The bisector of a triangle is a common geometric concept that does not cause much difficulty in learning. Having knowledge about its properties, you can solve many problems without much difficulty. What is a bisector? We will try to acquaint the reader with all the secrets of this mathematical line.

The essence of the concept

The name of the concept comes from the use of words in Latin, the meaning of which is “bi” - two, “sectio” - to cut. They specifically point to geometric meaning concepts - breaking up the space between rays into two equal parts.

The bisector of a triangle is a segment that originates from the vertex of the figure, and the other end is placed on the side that is located opposite it, while dividing the space into two identical parts.

To quickly associatively memorize mathematical concepts, many teachers use different terminology, which is reflected in poems or associations. Of course, using this definition is recommended for older children.

How is this line designated? Here we rely on the rules for designating segments or rays. If we're talking about about the designation of the angle bisector of a triangular figure, it is usually written as a segment whose ends are vertex and the point of intersection with the side opposite the vertex. Moreover, the beginning of the notation is written precisely from the vertex.

Attention! How many bisectors does a triangle have? The answer is obvious: as many as there are vertices - three.

Properties

Apart from the definition, not many properties of this geometric concept can be found in a school textbook. The first property of the bisector of a triangle that schoolchildren are introduced to is the inscribed center, and the second, directly related to it, is the proportionality of the segments. The bottom line is this:

1. Whatever the dividing line, there are points on it that are at the same distance from the sides, which make up the space between the rays.
2. In order to fit a circle into a triangular figure, it is necessary to determine the point at which these segments will intersect. This is the center point of the circle.
3. The parts of the side of a triangular geometric figure into which the dividing line divides it are located V proportional dependence from the sides forming the angle.

We will try to bring the remaining features into the system and present additional facts that will help to better understand the advantages of this geometric concept.

Length

One of the types of problems that cause difficulty for schoolchildren is finding the length of the bisector of an angle of a triangle. The first option, which contains its length, contains the following data:

• the amount of space between the rays from the vertex of which a given segment emerges;
• the lengths of the sides that form this angle.

To solve the problem formula used, the meaning of which is to find the ratio of the product of the values ​​of the sides that make up the angle, increased by 2 times, by the cosine of its half to the sum of the sides.

Let's look at a specific example. Suppose we are given a figure ABC, in which a segment is drawn from angle A and intersects side BC at point K. We denote the value of A as Y. Based on this, AK = (2*AB*AC*cos(Y/2))/(AB+ AC).

The second version of the problem, in which the length of the bisector of a triangle is determined, contains the following data:

• the meanings of all sides of the figure are known.

When solving a problem of this type, initially determine the semi-perimeter. To do this, you need to add up the values ​​of all sides and divide in half: p=(AB+BC+AC)/2. Next, we apply the computational formula that was used to determine the length of this segment in the previous problem. It is only necessary to make some changes to the essence of the formula in accordance with the new parameters. So, it is necessary to find the ratio of the doubled root of the second degree of the product of the lengths of the sides that are adjacent to the vertex by the semi-perimeter and the difference between the semi-perimeter and the length of the side opposite it to the sum of the sides that make up the angle. That is, AK = (2٦AB*AC*p*(p-BC))/(AB+AC).

Attention! To make it easier to master the material, you can turn to comic tales available on the Internet that tell about the “adventures” of this line.

Today will be a very easy lesson. We will consider just one object - the angle bisector - and prove its most important property, which will be very useful to us in the future.

Just don’t relax: sometimes students who want to get a high score on the same Unified State Exam or Unified State Exam cannot even accurately formulate the definition of a bisector in the first lesson.

And instead of doing really interesting tasks, we waste time on such simple things. So read, watch, and adopt it. :)

To begin with, a slightly strange question: what is an angle? That's right: an angle is simply two rays emanating from the same point. For example:

Examples of angles: acute, obtuse and right

As you can see from the picture, angles can be acute, obtuse, straight - it doesn’t matter now. Often, for convenience, an additional point is marked on each ray and they say that in front of us is the angle $AOB$ (written as $\angle AOB$).

Captain Obviousness seems to be hinting that in addition to the rays $OA$ and $OB$, it is always possible to draw a bunch of more rays from the point $O$. But among them there will be one special one - he is called a bisector.

Definition. The bisector of an angle is the ray that comes out from the vertex of that angle and bisects the angle.

For the above angles, the bisectors will look like this:

Examples of bisectors for acute, obtuse and right angle

Since in real drawings it is not always obvious that a certain ray (in our case it is the $OM$ ray) splits the original angle into two equal ones, in geometry it is customary to mark equal angles with the same number of arcs (in our drawing this is 1 arc for an acute angle, two for obtuse, three for straight).

Okay, we've sorted out the definition. Now you need to understand what properties the bisector has.

The main property of an angle bisector

In fact, the bisector has a lot of properties. And we will definitely look at them in the next lesson. But there is one trick that you need to understand right now:

Theorem. The bisector of an angle is the locus of points equidistant from the sides given angle.

Translated from mathematical into Russian, this means two facts at once:

1. Any point lying on the bisector of a certain angle is at the same distance from the sides of this angle.
2. And vice versa: if a point lies at the same distance from the sides of a given angle, then it is guaranteed to lie on the bisector of this angle.

Before proving these statements, let's clarify one point: what, exactly, is called the distance from a point to the side of an angle? Here the good old determination of the distance from a point to a line will help us:

Definition. The distance from a point to a line is the length of the perpendicular drawn from a given point to this line.

For example, consider a line $l$ and a point $A$ that does not lie on this line. Let us draw a perpendicular to $AH$, where $H\in l$. Then the length of this perpendicular will be the distance from point $A$ to straight line $l$.

Graphic representation of the distance from a point to a line

Since an angle is simply two rays, and each ray is a piece of a straight line, it is easy to determine the distance from a point to the sides of an angle. These are just two perpendiculars:

Determine the distance from the point to the sides of the angle

That's all! Now we know what a distance is and what a bisector is. Therefore, we can prove the main property.

As promised, we will split the proof into two parts:

1. The distances from the point on the bisector to the sides of the angle are the same

Consider an arbitrary angle with vertex $O$ and bisector $OM$:

Let us prove that this very point $M$ is at the same distance from the sides of the angle.

Proof. Let us draw perpendiculars from point $M$ to the sides of the angle. Let's call them $M((H)_(1))$ and $M((H)_(2))$:

Draw perpendiculars to the sides of the angle

We obtained two right triangles: $\vartriangle OM((H)_(1))$ and $\vartriangle OM((H)_(2))$. They have a common hypotenuse $OM$ and equal angles:

1. $\angle MO((H)_(1))=\angle MO((H)_(2))$ by condition (since $OM$ is a bisector);
2. $\angle M((H)_(1))O=\angle M((H)_(2))O=90()^\circ $ by construction;
3. $\angle OM((H)_(1))=\angle OM((H)_(2))=90()^\circ -\angle MO((H)_(1))$, since the sum The acute angles of a right triangle are always 90 degrees.

Consequently, the triangles are equal in side and two adjacent angles (see signs of equality of triangles). Therefore, in particular, $M((H)_(2))=M((H)_(1))$, i.e. the distances from point $O$ to the sides of the angle are indeed equal. Q.E.D.:)

2. If the distances are equal, then the point lies on the bisector

Now the situation is reversed. Let an angle $O$ be given and a point $M$ equidistant from the sides of this angle:

Let us prove that the ray $OM$ is a bisector, i.e. $\angle MO((H)_(1))=\angle MO((H)_(2))$.

Proof. First, let’s draw this very beam $OM$, otherwise there will be nothing to prove:

Conducted $OM$ beam inside the corner

Again we get two right triangles: $\vartriangle OM((H)_(1))$ and $\vartriangle OM((H)_(2))$. Obviously they are equal because:

1. Hypotenuse $OM$ - general;
2. Legs $M((H)_(1))=M((H)_(2))$ by condition (after all, the point $M$ is equidistant from the sides of the angle);
3. The remaining legs are also equal, because by the Pythagorean theorem $OH_(1)^(2)=OH_(2)^(2)=O((M)^(2))-MH_(1)^(2)$.

Therefore, the triangles $\vartriangle OM((H)_(1))$ and $\vartriangle OM((H)_(2))$ on three sides. In particular, their angles are equal: $\angle MO((H)_(1))=\angle MO((H)_(2))$. And this just means that $OM$ is a bisector.

To conclude the proof, we mark the resulting equal angles with red arcs:

The bisector splits the angle $\angle ((H)_(1))O((H)_(2))$ into two equal ones

As you can see, nothing complicated. We have proven that the bisector of an angle is the locus of points equidistant to the sides of this angle. :)

Now that we have more or less decided on the terminology, it’s time to move on to new level. In the next lesson we will look at more complex properties of the bisector and learn how to apply them to solve real problems.

PROPERTIES OF A BISSECTRIX

Bisector Property: In a triangle, the bisector divides the opposite side into segments proportional to the adjacent sides.

Bisector external corner The bisector of an external angle of a triangle intersects the extension of its side at a point, the distances from which to the ends of this side are proportional to the adjacent sides of the triangle, respectively. C B A D

Formulas for the length of a bisector:

Formula for finding the lengths of the segments into which the bisector divides the opposite side of the triangle

Formula for finding the ratio of the lengths of the segments into which the bisector is divided by the point of intersection of the bisectors

Problem 1. One of the bisectors of a triangle is divided by the intersection point of the bisectors in a ratio of 3:2, counting from the vertex. Find the perimeter of the triangle if the length of the side of the triangle to which this bisector is drawn is 12 cm.

Solution Let's use the formula to find the ratio of the lengths of the segments into which the bisector is divided by the point of intersection of the bisectors in the triangle:   a + c = = 18  P ∆ ABC = a + b + c = b +(a + c) = 12 + 18 = 30. Answer: P = 30cm.

Task 2. Bisectors BD and CE ∆ ABC intersect at point O. AB=14, BC=6, AC=10. Find O D.

Solution. Let's use the formula to find the length of the bisector: We have: BD = BD = = According to the formula for the ratio of the segments into which the bisector is divided by the point of intersection of the bisectors: l = . 2 + 1 = 3 parts total.

this is part 1  OD = Answer: OD =

Problems In ∆ ABC the bisectors AL and BK are drawn. Find the length of the segment KL if AB = 15, AK =7.5, BL = 5. At ∆ ABC there is a bisector AD, and through point D a line parallel to AC and intersecting AB at point E. Find the ratio of the areas ∆ ABC and ∆ BDE , if AB = 5, AC = 7. Find the bisectors of the acute angles of a right triangle with legs 24 cm and 18 cm. IN right triangle the bisector of an acute angle divides the opposite leg into segments 4 and 5 cm long. Determine the area of ​​the triangle.

5. B isosceles triangle the base and side are equal to 5 and 20 cm, respectively. Find the bisector of the angle at the base of the triangle. 6. Find the bisector of the right angle of a triangle whose legs are equal to a and b. 7. Calculate the length of the bisector of angle A of triangle ABC with side lengths a = 18 cm, b = 15 cm, c = 12 cm. 8. In triangle ABC, the lengths of sides AB, BC and AC are in the ratio 2:4:5, respectively. Find the ratio in which the bisectors are divided internal corners at the point of their intersection.

Answers: Answer: Answer: Answer: Answer: Answer: Answer: Answer: Answer: AP = 6 AP = 10 cm KL = CP =

Instructions

If a given triangle is isosceles or regular, then it has
two or three sides, then its bisector, according to the property triangle, will also be the median. And, therefore, the opposite one will be divided in half by the bisector.

Measure the opposite side with a ruler triangle, where the bisector will tend. Divide this side in half and place a dot in the middle of the side.

Draw a straight line passing through the constructed point and the opposite vertex. This will be the bisector triangle.

Sources:

• Medians, bisectors and altitudes of a triangle

Dividing an angle in half and calculating the length of a line drawn from its top to the opposite side is something that cutters, surveyors, installers and people of some other professions need to be able to do.

You will need

• Tools Pencil Ruler Protractor Sine and Cosine Tables Mathematical formulas and concepts: Definition of a bisector Theorems of sines and cosines Bisector theorem

Instructions

Construct a triangle of the required size, depending on what is given to you? dfe sides and the angle between them, three sides or two angles and the side located between them.

Label the vertices of the corners and sides with the traditional Latin letters A, B and C. The vertices of the corners are denoted by , and the opposite sides are denoted by lowercase letters. Label the angles with Greek letters?,? And?

Using the theorems of sines and cosines, calculate the angles and sides triangle.

Remember bisectors. Bisector - dividing an angle in half. Angle bisector triangle divides the opposite into two segments, which are equal to the ratio of the two adjacent sides triangle.

Draw the bisectors of the angles. Label the resulting segments with the names of the angles, written in lowercase letters, with a subscript l. Side c is divided into segments a and b with indices l.

Calculate the lengths of the resulting segments using the law of sines.

Video on the topic

note

The length of the segment, which is simultaneously the side of the triangle formed by one of the sides of the original triangle, the bisector and the segment itself, is calculated using the law of sines. In order to calculate the length of another segment of the same side, use the ratio of the resulting segments and the adjacent sides of the original triangle.

Helpful advice

To avoid confusion, draw bisectors different angles different colors.

Bisector angle called a ray that starts at the vertex angle and divides it into two equal parts. Those. to spend bisector, you need to find the middle angle. The easiest way to do this is with a compass. In this case, you do not need to do any calculations, and the result will not depend on whether the quantity is angle an integer.

You will need

• compass, pencil, ruler.

Instructions

Leaving the width of the compass opening the same, place the needle at the end of the segment on one of the sides and draw part of the circle so that it is located inside angle. Do the same with the second one. You will end up with two parts of circles that will intersect inside angle- approximately in the middle. Parts of circles can intersect at one or two points.

Video on the topic

Helpful advice

To construct the bisector of an angle, you can use a protractor, but this method requires greater accuracy. Moreover, if the angle value is not an integer, the probability of errors in constructing the bisector increases.

When building or developing home design projects, it is often necessary to build corner, equal to what is already available. Templates and school knowledge of geometry come to the rescue.

Instructions

An angle is formed by two straight lines emanating from one point. This point will be called the vertex of the angle, and the lines will be the sides of the angle.

Use three to indicate corners: one at the top, two at the sides. Called corner, starting with the letter that stands on one side, then the letter that stands at the top is called, and then the letter on the other side. Use others to indicate angles if you prefer otherwise. Sometimes only one letter is named, which is at the top. And you can denote angles with Greek letters, for example, α, β, γ.

There are situations when it is necessary corner, so that it is narrower than the given corner. If it is not possible to use a protractor when constructing, you can only get by with a ruler and a compass. Suppose, on a straight line marked with the letters MN, you need to construct corner at point K, so that it is equal to angle B. That is, from point K it is necessary to draw a straight line with line MN corner, which will be equal to angle B.

First, mark a point on each side of a given angle, for example, points A and C, then connect points C and A with a straight line. Get tre corner nik ABC.

Now build the same tre on the straight line MN corner so that its vertex B is on the line at point K. Use the rule for constructing a triangle corner nnik in three. Lay off the segment KL from point K. It must be equal to the segment BC. Get the L point.

From point K, draw a circle with a radius equal to segment BA. From L, draw a circle with radius CA. Connect the resulting point (P) of intersection of two circles with K. Get three corner KPL, which will be equal to three corner ABC book. This is how you get corner K. It will be equal to angle B. To make it more convenient and faster, set off equal segments from vertex B, using one compass opening, without moving the legs, describe a circle with the same radius from point K.

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Tip 5: How to construct a triangle using two sides and a median

Triangle is the simplest geometric figure, having three vertices connected in pairs by segments that form the sides of this polygon. The segment connecting the vertex to the middle of the opposite side is called the median. Knowing the lengths of two sides and the median connecting at one of the vertices, you can construct a triangle without having information about the length of the third side or the size of the angles.

Instructions

Draw a segment from point A the length of which is one of the known sides of the triangle (a). Mark the end point of this segment with the letter B. After this, one of the sides (AB) of the desired triangle can already be considered constructed.

Using a compass, draw a circle with a radius equal to twice the length of the median (2∗m) and with a center at point A.

Using a compass, draw a second circle with a radius equal to length known party(b), and with the center at point B. Put the compass aside for a while, but leave the measured one on it - you will need it again a little later.

Construct a line segment connecting point A to the intersection point of the two you drew. Half of this segment will be the one you are building - measure this half and put point M. At this moment you have one side of the desired triangle (AB) and its median (AM).

Using a compass, draw a circle with a radius equal to the length of the second known side (b) and with a center at point A.

Draw a segment that should start at point B, pass through point M and end at the point of intersection of the straight line with the circle you drew in the previous step. Designate the point of intersection with the letter C. Now the side BC, unknown according to the conditions of the problem, has been constructed in the required one.

The ability to divide any angle with a bisector is needed not only to get an “A” in mathematics. This knowledge will be very useful for builders, designers, surveyors and dressmakers. In life, you need to be able to divide many things in half.

Everyone at school learned a joke about a rat that runs around corners and divides the corner in half. The name of this nimble and intelligent rodent was Bisector. It is not known how the rat divided the corner, but the following methods can be suggested for mathematicians in the school textbook “Geometry”.

Using a protractor

The easiest way to conduct a bisector is using a device for. You need to attach the protractor to one side of the angle, aligning the reference point with its tip O. Then measure the angle in degrees or radians and divide it by two. Using the same protractor, set aside the obtained degrees from one of the sides and draw a straight line, which will become a bisector, to the starting point of angle O.

Using a compass

You need to take a compass and move it to any arbitrary size (within the limits of the drawing). Having placed the tip at the starting point of angle O, draw an arc intersecting the rays, marking two points on them. They are designated A1 and A2. Then, placing the compass alternately at these points, you should draw two circles of the same arbitrary diameter (on the scale of the drawing). Their intersection points are designated C and B. Next, you need to draw a straight line through points O, C and B, which will be the desired bisector.

Using a ruler

In order to draw the bisector of an angle using a ruler, you need to lay off segments of the same length from point O on the rays (sides) and designate them as points A and B. Then you should connect them with a straight line and, using a ruler, divide the resulting segment in half, designating point C. A bisector will be obtained if you draw a straight line through points C and O.

No tools

If not measuring instruments, you can use your ingenuity. It is enough to simply draw an angle on tracing paper or ordinary thin paper and carefully fold the piece of paper so that the rays of the angle align. The fold line in the drawing will be the desired bisector.

Straight angle

An angle greater than 180 degrees can be divided by a bisector using the same methods. Only it will be necessary to divide not it, but the adjacent sharp corner, remaining from the circle. The continuation of the found bisector will become the desired straight line, dividing the unfolded angle in half.

Angles in a triangle

It should be remembered that in an equilateral triangle the bisector is also the median and altitude. Therefore, the bisector in it can be found by simply lowering the perpendicular to the side opposite the angle (height) or dividing this side in half and connecting the midpoint with the opposite angle (median).

Video on the topic

The mnemonic rule “a bisector is a rat that runs around the corners and divides them in half” describes the essence of the concept, but does not provide recommendations for constructing a bisector. To draw it, in addition to the rule, you will need a compass and a ruler.

Instructions

Let's say you need to build bisector angle A. Take a compass, place its tip at point A (angle) and draw a circle of any . Where it intersects the sides of the corner, place points B and C.

Measure the radius of the first circle. Draw another one with the same radius, placing a compass at point B.

Draw the next circle (equal in size to the previous ones) with its center at point C.

All three circles must intersect at one point - let's call it F. Using a ruler, draw a ray passing through points A and F. This will be the desired bisector of angle A.

There are several rules that will help you find. For example, it is the opposite in, equal to the ratio two adjacent sides. In isosceles