In what ratio are the altitudes of a triangle divided? Basic elements of triangle abc

E A → ⋅ B C → + E B → ⋅ C A → + E C → ⋅ A B → = 0 (\displaystyle (\overrightarrow (EA))\cdot (\overrightarrow (BC))+(\overrightarrow (EB))\cdot (\ overrightarrow (CA))+(\overrightarrow (EC))\cdot (\overrightarrow (AB))=0)

(To prove the identity, you should use the formulas

A B → = E B → − E A → , B C → = E C → − E B → , C A → = E A → − E C → (\displaystyle (\overrightarrow (AB))=(\overrightarrow (EB))-(\overrightarrow (EA )),\,(\overrightarrow (BC))=(\overrightarrow (EC))-(\overrightarrow (EB)),\,(\overrightarrow (CA))=(\overrightarrow (EA))-(\overrightarrow (EC)))

Point E should be taken as the intersection of two altitudes of the triangle.)

• Orthocenter isogonally conjugate to the center circumcircle .
• Orthocenter lies on the same line as the centroid, the center circumcircle and the center of the circle of nine points (see Euler's straight line).
• Orthocenter of an acute triangle is the center of the circle inscribed in its orthotriangle.
• The center of a triangle described by the orthocenter with vertices at the midpoints of the sides of the given triangle. The last triangle is called the complementary triangle to the first triangle.
• The last property can be formulated as follows: The center of the circle circumscribed about the triangle serves orthocenter additional triangle.
• Points, symmetrical orthocenter of a triangle with respect to its sides lie on the circumcircle.
• Points, symmetrical orthocenter triangles relative to the midpoints of the sides also lie on the circumscribed circle and coincide with points diametrically opposite to the corresponding vertices.
• If ABOUT is the center of the circumcircle ΔABC, then O H → = O A → + O B → + O C → (\displaystyle (\overrightarrow (OH))=(\overrightarrow (OA))+(\overrightarrow (OB))+(\overrightarrow (OC))) ,
• The distance from the vertex of the triangle to the orthocenter is twice as great as the distance from the center of the circumcircle to the opposite side.
• Any segment drawn from orthocenter before the intersection with the circumcircle, it is always bisected by the Euler circle. Orthocenter is the homothety center of these two circles.
• Hamilton's theorem. Three line segments connecting the orthocenter to the vertices of the acute triangle split it into three triangles having the same Euler circle (circle of nine points) as the original acute triangle.
• Corollaries of Hamilton's theorem:
  • Three straight line segments connecting the orthocenter with the vertices of an acute triangle divide it into three Hamilton triangle having equal radii of circumscribed circles.
  • The radii of circumscribed circles of three Hamilton triangles equal to the radius of the circle circumscribed about the original acute triangle.
• In an acute triangle, the orthocenter lies inside the triangle; in an obtuse angle - outside the triangle; in a rectangular - at the top right angle.

Properties of altitudes of an isosceles triangle

• If two altitudes in a triangle are equal, then the triangle is isosceles (the Steiner-Lemus theorem), and the third altitude is both the median and the bisector of the angle from which it emerges.
• The converse is also true: in an isosceles triangle, two altitudes are equal, and the third altitude is both the median and the bisector.
• An equilateral triangle has all three heights equal.

Properties of the bases of altitudes of a triangle

• Grounds heights form a so-called orthotriangle, which has its own properties.
• The circle circumscribed about an orthotriangle is the Euler circle. This circle also contains three midpoints of the sides of the triangle and three midpoints of three segments connecting the orthocenter with the vertices of the triangle.
• Another formulation of the last property:
  • Euler's theorem for the nine-point circle. Grounds three heights arbitrary triangle, the midpoints of its three sides ( the foundations of its internal medians) and the midpoints of three segments connecting its vertices with the orthocenter, all lie on the same circle (on nine point circle).
• Theorem. In any triangle, the segment connecting grounds two heights triangle, cuts off a triangle similar to the given one.
• Theorem. In a triangle, the segment connecting grounds two heights triangles lying on two sides antiparallel to a third party with whom he has no common points. A circle can always be drawn through its two ends, as well as through the two vertices of the third mentioned side.

Other properties of triangle altitudes

Properties of the minimum altitude of a triangle

The minimum altitude of a triangle has many extreme properties. For example:

• The minimum orthogonal projection of a triangle onto lines lying in the plane of the triangle has a length equal to the smallest of its altitudes.
• The minimum straight cut in a plane through which a rigid triangular plate can be pulled must have a length equal to the smallest of the heights of this plate.
• With the continuous movement of two points along the perimeter of the triangle towards each other, the maximum distance between them during the movement from the first meeting to the second cannot be less than the length of the smallest height of the triangle.
• The minimum height in a triangle always lies within that triangle.

Basic relationships

• h a = b sin ⁡ γ = c sin ⁡ β , (\displaystyle h_(a)=b\sin \gamma =c\sin \beta ,)
• h a = 2 S a , (\displaystyle h_(a)=(\frac (2S)(a)),) Where S (\displaystyle S)- area of ​​a triangle, a (\displaystyle a)- the length of the side of the triangle by which the height is lowered.
• h a 2 = 1 2 (b 2 + c 2 − 1 2 (a 2 + (b 2 − c 2) 2 a 2)) (\displaystyle h_(a)^(2)=(\frac (1)(2 ))(b^(2)+c^(2)-(\frac (1)(2))(a^(2)+(\frac ((b^(2)-c^(2))^ (2))(a^(2))))))
• h a = b c 2 R , (\displaystyle h_(a)=(\frac (bc)(2R)),) Where b c (\displaystyle bc)- product of the sides, R − (\displaystyle R-) circumscribed circle radius
• h a: h b: h c = 1 a: 1 b: 1 c = b c: a c: a b (\displaystyle h_(a):h_(b):h_(c)=(\frac (1)(a)):( \frac (1)(b)):(\frac (1)(c))=bc:ac:ab)
• 1 h a + 1 h b + 1 h c = 1 r (\displaystyle (\frac (1)(h_(a)))+(\frac (1)(h_(b)))+(\frac (1)(h_ (c)))=(\frac (1)(r))), Where r (\displaystyle r)- radius of the inscribed circle.
• S = 1 (1 h a + 1 h b + 1 h c) ⋅ (1 h a + 1 h b − 1 h c) ⋅ (1 h a + 1 h c − 1 h b) ⋅ (1 h b + 1 h c − 1 h a) (\displaystyle S =(\frac (1)(\sqrt (((\frac (1)(h_(a)))+(\frac (1)(h_(b)))+(\frac (1)(h_(c ))))(\cdot )((\frac (1)(h_(a)))+(\frac (1)(h_(b)))-(\frac (1)(h_(c))) )(\cdot )((\frac (1)(h_(a)))+(\frac (1)(h_(c)))-(\frac (1)(h_(b))))(\ cdot )((\frac (1)(h_(b)))+(\frac (1)(h_(c)))-(\frac (1)(h_(a)))))))), Where S (\displaystyle S) - area of ​​a triangle.
• a = 2 h a ⋅ (1 h a + 1 h b + 1 h c) ⋅ (1 h a + 1 h b − 1 h c) ⋅ (1 h a + 1 h c − 1 h b) ⋅ (1 h b + 1 h c − 1 h a) (\ displaystyle a=(\frac (2)(h_(a)(\cdot )(\sqrt (((\frac (1)(h_(a)))+(\frac (1)(h_(b))) +(\frac (1)(h_(c))))(\cdot )((\frac (1)(h_(a)))+(\frac (1)(h_(b)))-(\ frac (1)(h_(c))))(\cdot )((\frac (1)(h_(a)))+(\frac (1)(h_(c)))-(\frac (1 )(h_(b))))(\cdot )((\frac (1)(h_(b)))+(\frac (1)(h_(c)))-(\frac (1)(h_ (a))))))))), a (\displaystyle a)- the side of the triangle to which the height descends h a (\displaystyle h_(a)).
• Height of an isosceles triangle lowered to the base: h c = 1 2 4 a 2 − c 2 , (\displaystyle h_(c)=(\frac (1)(2))(\sqrt (4a^(2)-c^(2))),)

Where c (\displaystyle c)- base, a (\displaystyle a)- side.

Right Triangle Altitude Theorem

If the height is right triangle A B C (\displaystyle ABC) length h (\displaystyle h) drawn from the vertex of a right angle, divides the hypotenuse with length c (\displaystyle c) into segments m (\displaystyle m) And n (\displaystyle n), corresponding to the legs b (\displaystyle b) And a (\displaystyle a), then the following equalities are true.

Triangle - a polygon with three sides, or a closed broken line with three links, or a figure formed by three segments connecting three points that do not lie on the same straight line (see Fig. 1).

Basic elements of triangle abc

Peaks – points A, B, and C;

Parties – segments a = BC, b = AC and c = AB connecting the vertices;

Angles – α, β, γ formed by three pairs of sides. Angles are often designated in the same way as vertices, with the letters A, B, and C.

The angle formed by the sides of a triangle and lying in its interior area is called an interior angle, and the one adjacent to it is the adjacent angle of the triangle (2, p. 534).

Heights, medians, bisectors and midlines of a triangle

In addition to the main elements in a triangle, other segments with interesting properties are also considered: heights, medians, bisectors and midlines.

Height

Triangle heights- these are perpendiculars dropped from the vertices of the triangle to opposite sides.

To plot the height, you must perform the following steps:

1) draw a straight line containing one of the sides of the triangle (if the height is drawn from the vertex of an acute angle in an obtuse triangle);

2) from the vertex lying opposite the drawn line, draw a segment from the point to this line, making an angle of 90 degrees with it.

The point of intersection of the altitude with the side of the triangle is called height base (see Fig. 2).

Properties of triangle altitudes

In a right triangle, the altitude drawn from the vertex of the right angle splits it into two triangles similar to the original triangle.

In an acute triangle, its two altitudes cut off similar triangles from it.

If the triangle is acute, then all the bases of the altitudes belong to the sides of the triangle, and in an obtuse triangle, two altitudes fall on the continuation of the sides.

Three altitudes in an acute triangle intersect at one point and this point is called orthocenter triangle.

Median

Medians(from Latin mediana – “middle”) - these are segments connecting the vertices of the triangle with the midpoints of the opposite sides (see Fig. 3).

To construct the median, you must perform the following steps:

1) find the middle of the side;

2) connect the point that is the middle of the side of the triangle with the opposite vertex with a segment.

Properties of triangle medians

The median divides a triangle into two triangles of equal area.

The medians of a triangle intersect at one point, which divides each of them in a ratio of 2:1, counting from the vertex. This point is called center of gravity triangle.

The entire triangle is divided by its medians into six equal triangles.

Bisector

Bisectors(from Latin bis - twice and seko - cut) are the straight line segments enclosed inside a triangle that bisect its angles (see Fig. 4).

To construct a bisector, you must perform the following steps:

1) construct a ray coming out from the vertex of the angle and dividing it into two equal parts (the bisector of the angle);

2) find the point of intersection of the bisector of the angle of the triangle with the opposite side;

3) select a segment connecting the vertex of the triangle with the intersection point on the opposite side.

Properties of triangle bisectors

The bisector of an angle of a triangle divides the opposite side in the ratio equal to the ratio two adjacent sides.

The bisectors of the interior angles of a triangle intersect at one point. This point is called the center of the inscribed circle.

The bisectors of the internal and external angles are perpendicular.

If the bisector of an exterior angle of a triangle intersects the extension of the opposite side, then ADBD=ACBC.

The bisectors of one internal and two external angles of a triangle intersect at one point. This point is the center of one of the three excircles of this triangle.

The bases of the bisectors of two interior and one exterior angles of a triangle lie on the same straight line if the bisector of the exterior angle is not parallel to the opposite side of the triangle.

If the bisectors of the external angles of a triangle are not parallel to opposite sides, then their bases lie on the same straight line.

Properties

• The altitudes of a triangle intersect at one point, called the orthocenter. - This statement is easy to prove using a vector identity that is valid for any points A, B, C, E, not necessarily even those lying in the same plane:

(To prove the identity, you should use the formulas

Point E should be taken as the intersection of two altitudes of the triangle.)

• In a right triangle, the altitude drawn from the vertex of the right angle splits it into two triangles similar to the original one.
• In an acute triangle, its two altitudes cut off similar triangles from it.
• The bases of the heights form a so-called orthotriangle, which has its own properties.

The minimum altitude of a triangle has many extreme properties. For example:

• The minimum orthogonal projection of a triangle onto lines lying in the plane of the triangle has a length equal to the smallest of its altitudes.

• The minimum straight cut in a plane through which a rigid triangular plate can be pulled must have a length equal to the smallest of the heights of this plate.

• With the continuous movement of two points along the perimeter of the triangle towards each other, the maximum distance between them during the movement from the first meeting to the second cannot be less than the length of the smallest height of the triangle.

The minimum height in a triangle always lies within that triangle.

Basic relationships

where is the area of ​​the triangle, is the length of the side of the triangle by which the height is lowered.

where is the base.

Right Triangle Altitude Theorem

If a height of length h drawn from the vertex of a right angle divides the hypotenuse of length c into segments m and n corresponding to b and a, then the following equalities are true:

Mnemonic poem

The height is like a cat, Which, arching its back, And at right angles Connects the top And the side with its tail.

see also

Links

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2010.

See what “Height of a triangle” is in other dictionaries: HEIGHT, heights, plural. heights, heights, women 1. units only Extension from bottom to top, height. Height of the house. Tower high altitude . || (pl. only special scientific). Distance from earth's surface , measured along a vertical line from bottom to top. The airplane was flying... Dictionary

Ushakova This term has other meanings, see Height (meanings). Height in elementary geometry perpendicular segment dropped from a vertex geometric figure

(for example, a triangle, a pyramid, a cone) on its base or on ... ... Wikipedia height - ы/; pl. height; and. see also high-rise, high-rise 1) Size, length of something. from the bottom to the top, from bottom to top. Height/ of a house, tree, mountain. Height/waves. The dam is one hundred five feet high...

Dictionary of many expressions Y; pl. heights; and. 1. Size, length of something. from the bottom to the top, from bottom to top. V. houses, trees, mountains. V. waves. The dam is one hundred and fifty meters high. Measure, determine the height of something. 2. Distance from which l. surface to... ...

encyclopedic Dictionary height of original thread triangle - (H) The distance between the apex and base of the original thread triangle in a direction perpendicular to the axis of the thread. [GOST 11708 82 (ST SEV 2631 80)] Topics of the interchangeability standard General terms basic elements and thread parameters EN ... ...

Technical Translator's Guide

HEIGHT, in geometry, a perpendicular segment descended from the top of a geometric figure (e.g., triangle, pyramid, cone) to its base (or continuation of the base), as well as the length of this segment. The height of a prism, cylinder, spherical layer, and... ... Y; pl. heights; and. 1. Size, length of something. from the bottom to the top, from bottom to top. V. houses, trees, mountains. V. waves. The dam is one hundred and fifty meters high. Measure, determine the height of something. 2. Distance from which l. surface to... ...

In geometry, a perpendicular segment drawn from the vertex of a geometric figure (e.g., triangle, pyramid, cone) to its base (or continuation of the base), as well as the length of this segment. The height of the prism, cylinder, spherical layer, as well as... ... Big Encyclopedic Dictionary

HEIGHT, s, plural. from, from, from, wives. 1. Size, length of something. from the bottom to the top. B. brickwork. V. surf. V. cyclone. 2. Space, distance from the ground upward. Look up. The plane is gaining altitude. Fly to... ... Ozhegov's Explanatory Dictionary

Height in geometry, a perpendicular segment descended from the top of a geometric figure (for example, a triangle, pyramid, cone) to its base or continuation of the base, as well as the length of this segment. V. prism, cylinder, spherical layer,... ... Great Soviet Encyclopedia

When solving geometric problems, it is useful to follow such an algorithm. While reading the conditions of the problem, it is necessary

• Make a drawing. The drawing should correspond as much as possible to the conditions of the problem, so its main task is to help find the solution
• Put all the data from the problem statement on the drawing
• Write everything out geometric concepts, which appear in the problem
• Remember all the theorems that relate to these concepts
• Draw on the drawing all the relationships between the elements of a geometric figure that follow from these theorems

For example, if the problem contains the words bisector of an angle of a triangle, you need to remember the definition and properties of a bisector and indicate equal or proportional segments and angles in the drawing.

In this article you will find the basic properties of a triangle that you need to know to successfully solve problems.

TRIANGLE.

Area of ​​a triangle.

1. ,

here - an arbitrary side of the triangle, - the height lowered to this side.

2. ,

here and are arbitrary sides of the triangle, and is the angle between these sides:

3. Heron's formula:

Here are the lengths of the sides of the triangle, is the semi-perimeter of the triangle,

4. ,

here is the semi-perimeter of the triangle, and is the radius of the inscribed circle.

Let be the lengths of the tangent segments.

Then Heron's formula can be written as follows:

5.

6. ,

here - the lengths of the sides of the triangle, - the radius of the circumscribed circle.

If a point is taken on the side of a triangle that divides this side in the ratio m: n, then the segment connecting this point with the vertex of the opposite angle divides the triangle into two triangles, the areas of which are in the ratio m: n:

The ratio of the areas of similar triangles is equal to the square of the similarity coefficient.

Median of a triangle

This is a segment connecting the vertex of a triangle to the middle of the opposite side.

Medians of a triangle intersect at one point and are divided by the intersection point in a ratio of 2:1, counting from the vertex.

Median intersection point regular triangle divides the median into two segments, the smaller of which is equal to the radius of the inscribed circle, and the larger of which is equal to the radius of the circumscribed circle.

The radius of the circumscribed circle is twice the radius of the inscribed circle: R=2r

Median length arbitrary triangle

,

here - the median drawn to the side - the lengths of the sides of the triangle.

Bisector of a triangle

This is the bisector segment of any angle of a triangle connecting the vertex of this angle with the opposite side.

Bisector of a triangle divides a side into segments proportional to the adjacent sides:

Bisectors of a triangle intersect at one point, which is the center of the inscribed circle.

All points of the angle bisector are equidistant from the sides of the angle.

Triangle height

This is a perpendicular segment dropped from the vertex of the triangle to the opposite side, or its continuation. In an obtuse triangle, the altitude drawn from the vertex of the acute angle lies outside the triangle.

The altitudes of a triangle intersect at one point, which is called orthocenter of the triangle.

To find the height of a triangle drawn to the side, you need to find its area in any available way, and then use the formula:

Center of the circumcircle of a triangle, lies at the intersection point of the perpendicular bisectors drawn to the sides of the triangle.

Circumference radius of a triangle can be found using the following formulas:

Here are the lengths of the sides of the triangle, and is the area of ​​the triangle.

,

where is the length of the side of the triangle and is the opposite angle. (This formula follows from the sine theorem.)

Triangle inequality

Each side of the triangle is less than the sum and greater than the difference of the other two.

The sum of the lengths of any two sides is always greater than the length of the third side:

Opposite the larger side lies the larger angle; Opposite the larger angle lies the larger side:

If , then vice versa.

Theorem of sines:

The sides of a triangle are proportional to the sines of the opposite angles:

Cosine theorem:

square side of triangle equal to the sum squares of the other two sides without twice the product of these sides by the cosine of the angle between them:

Right triangle

- This is a triangle, one of the angles of which is 90°.

Sum sharp corners of a right triangle is 90°.

The hypotenuse is the side that lies opposite the 90° angle. The hypotenuse is the longest side.

Pythagorean theorem:

the square of the hypotenuse is equal to the sum of the squares of the legs:

The radius of a circle inscribed in a right triangle is equal to

,

here is the radius of the inscribed circle, - the legs, - the hypotenuse:

Center of the circumcircle of a right triangle lies in the middle of the hypotenuse:

Median of a right triangle drawn to the hypotenuse, is equal to half the hypotenuse.

Definition of sine, cosine, tangent and cotangent of a right triangle look

The ratio of elements in a right triangle:

The square of the altitude of a right triangle drawn from the vertex of a right angle is equal to the product of the projections of the legs onto the hypotenuse:

The square of the leg is equal to the product of the hypotenuse and the projection of the leg onto the hypotenuse:

Leg lying opposite the corner equal to half the hypotenuse:

Isosceles triangle.

Bisector isosceles triangle drawn to the base is the median and height.

In an isosceles triangle, the base angles are equal.

Apex angle.

And - sides,

And - angles at the base.

Height, bisector and median.

Attention! The height, bisector and median drawn to the side do not coincide.

Regular triangle

(or equilateral triangle ) is a triangle, all sides and angles of which are equal to each other.

Area of ​​a regular triangle equal to

where is the length of the side of the triangle.

Center of a circle inscribed in a regular triangle, coincides with the center of the circle circumscribed about a regular triangle and lies at the point of intersection of the medians.

Intersection point of the medians of a regular triangle divides the median into two segments, the smaller of which is equal to the radius of the inscribed circle, and the larger of which is equal to the radius of the circumscribed circle.

If one of the angles of an isosceles triangle is 60°, then the triangle is regular.

Middle line of the triangle

This is a segment connecting the midpoints of two sides.

In the figure DE is the middle line of triangle ABC.

The middle line of the triangle is parallel to the third side and equal to its half: DE||AC, AC=2DE

External angle of a triangle

This is the angle adjacent to any angle of the triangle.

An exterior angle of a triangle is equal to the sum of two angles not adjacent to it.

External angle trigonometric functions:

Signs of equality of triangles:

1 . If two sides and the angle between them of one triangle are respectively equal to two sides and the angle between them of another triangle, then such triangles are congruent.

2 . If a side and two adjacent angles of one triangle are respectively equal to a side and two adjacent angles of another triangle, then such triangles are congruent.

3 If three sides of one triangle are respectively equal to three sides of another triangle, then such triangles are congruent.

Important: since in a right triangle two angles are obviously equal, then for equality of two right triangles equality of only two elements is required: two sides, or a side and an acute angle.

Signs of similarity of triangles:

1 . If two sides of one triangle are proportional to two sides of another triangle, and the angles between these sides are equal, then these triangles are similar.

2 . If three sides of one triangle are proportional to three sides of another triangle, then the triangles are similar.

3 . If two angles of one triangle are equal to two angles of another triangle, then the triangles are similar.

Important: V similar triangles similar sides lie opposite equal angles.

Menelaus' theorem

Let a line intersect a triangle, and is the point of its intersection with side , is the point of its intersection with side , and is the point of its intersection with the continuation of side . Then

Right Triangle Altitude Theorem

If the altitude in a right triangle ABC of length , drawn from the vertex of the right angle, divides the hypotenuse of length and into segments and corresponding to the legs and , then the following equalities are true:

·

·

Properties of the bases of altitudes of a triangle

· Grounds heights form a so-called orthotriangle, which has its own properties.

· The circle circumscribed about an orthotriangle is the Euler circle. This circle also contains three midpoints of the sides of the triangle and three midpoints of three segments connecting the orthocenter with the vertices of the triangle.

Another formulation of the last property:

· Euler's theorem for the nine-point circle.

Grounds three heights arbitrary triangle, the midpoints of its three sides ( the foundations of its internal medians) and the midpoints of three segments connecting its vertices with the orthocenter, all lie on the same circle (on nine point circle).

· Theorem. In any triangle, the segment connecting grounds two heights triangle, cuts off a triangle similar to the given one.

· Theorem. In a triangle, the segment connecting grounds two heights triangles lying on two sides antiparallel to a third party with whom he has no common ground. A circle can always be drawn through its two ends, as well as through the two vertices of the third mentioned side.

Other properties of triangle altitudes

· If the triangle versatile (scalene), then it internal the bisector drawn from any vertex lies between internal median and height drawn from the same vertex.

The height of a triangle is isogonally conjugate to the diameter (radius) circumcircle, drawn from the same vertex.

· In an acute triangle there are two heights cut off similar triangles from it.

· In a right triangle (for example, a triangle, a pyramid, a cone) on its base or on ... ... Wikipedia drawn from the vertex of a right angle, splits it into two triangles similar to the original one.

Properties of the minimum altitude of a triangle

The minimum altitude of a triangle has many extreme properties. For example:

· The minimum orthogonal projection of a triangle onto lines lying in the plane of the triangle has a length equal to the smallest of its altitudes.

· The minimum straight cut in the plane through which a rigid triangular plate can be pulled must have a length equal to the smallest of the heights of this plate.

· When two points move continuously along the perimeter of a triangle towards each other, the maximum distance between them during the movement from the first meeting to the second cannot be less than the length of the smallest height of the triangle.

· The minimum height in a triangle always lies inside that triangle.

Basic relationships

· where is the area of ​​the triangle, is the length of the side of the triangle by which the height is lowered.

· where is the product of the sides, the radius of the circumscribed circle

· ,

where is the radius of the inscribed circle.

Where is the area of ​​the triangle.

where is the side of the triangle to which the height descends.

· Height of an isosceles triangle lowered to the base:

where is the base.

· - height in an equilateral triangle.

Medians and altitudes in an equilateral triangle

The medians of a triangle intersect at one point, which divides each of them in a ratio of 2:1, counting from the vertex. This point is called center of gravity triangle. And in equilateral triangles, medians and altitudes are the same thing.

Consider an arbitrary triangle ABC. Let us denote by the letter O the point of intersection of its medians AA1 and BB1 and draw midline A1B1 of this triangle The medians of the triangle intersect at one point. The segment A1B1 is parallel to side AB, therefore angles 1 and 2, as well as angles 3 and 4, are equal as crosswise angles when parallel lines AB and A1B1 intersect with secants AA1 and BB1. Therefore, triangles AOB and A1OB1 are similar in two angles, and therefore their sides are proportional: AOA1O=BOB1O=ABA1B1. But AB=2⋅A1B1, so AO=2⋅A1O and BO=2⋅B1O. Thus, the intersection point O of the medians AA1 and BB1 divides each of them in a ratio of 2:1, counting from the vertex. Similarly, it is proved that the point of intersection of the medians BB1 and CC1 divides each of them in the ratio 2:1 counting from the vertex, and therefore coincides with the point O. Thus, all three medians of the triangle ABC intersect at the point O and are divided by it in the ratio 2: 1, counting from the top.

The theorem has been proven.

Let's imagine that at the vertices of the angle m₁=1, then at points A₁,B₁,C₁, m₂=2, since they are the midpoints of the sides. And here you can notice that the segments AA₁,BB₁,CC₁, which intersect at one point, are similar to levers with a fulcrum O, where AO-l₁, and OA₁-l₂ (shoulders). And by physical formula F₁/F₂=l₁/l₂, where F=m*g, where g-const, and it is reduced accordingly, it turns out m₁/m₂=l₁/l₂ i.e. ½=1/2.

The theorem has been proven.

Orthotriangle

Properties:

· Three altitudes of a triangle intersect at one point, this point is called the orthocenter

· Two adjacent sides orthotriangle form equal angles with the corresponding side of the original triangle

The altitudes of a triangle are the bisectors of an orthotriangle

An orthotriangle is the triangle with the smallest perimeter that can be inscribed in given triangle(Fagnano problem)

· The perimeter of an orthotriangle is equal to twice the product of the height of the triangle and the sine of the angle from which it originates.

· If points A 1 , B 1 and C 1 on sides BC, AC and AB of acute triangle ABC, respectively, are such that

then is an orthotriangle of triangle ABC.

Orthotriangle cuts off triangles similar to this one

Theorem on the property of bisectors of an orthotriangle

∟ B₁C₁C=∟B₁BC=∟CAA₁=∟CC₁A

CC₁-bisector ∟B₁C₁A

AA₁-bisector ∟B₁A₁C₁

BB₁-bisector ∟A₁B₁C₁