Class 10 Exam  >  Class 10 Notes  >  Mathematics (Maths) Class 10  >  Chapter Notes: Triangles

Triangles Class 10 Notes Maths Chapter 6

Introduction


In this chapter, we delve into the fascinating realm of figures that share the same shape but may differ in size - these are aptly termed similar figures. Building upon the groundwork laid in Class IX regarding the congruence of triangles, we now explore the concept of similarity. Unlike congruent figures that possess both the same shape and size, similar figures exhibit identical shapes while allowing for variations in size. 

Similar Figures

  • Two geometrical figures are said to be similar figures if they have the same shape but not necessarily the same size.
    Or
    A shape is said to be similar to other if the ratio of their corresponding sides is equal and the corresponding angles are equal.
  • Two polygons having the same number of sides are similar, if:
    (i) all the corresponding angles are equal and
    (ii) all the corresponding sides are in the same ratio or proportion.
    Triangles Class 10 Notes Maths Chapter 6If only one condition from (i) and (ii) is true for two polygons, then they cannot be similar.

Similarity of Triangles

Two triangles are said to be similar triangles if:

  • Their corresponding angles are equal and 
  • their corresponding sides are proportional (i.e., the ratios between the lengths of corresponding sides are equal).  

For example:
If in ∆ABC and ∆PQR
Triangles Class 10 Notes Maths Chapter 6∠A = ∠P, ∠B = ∠Q, ∠C = ∠R and,
AB/PQ = BC/QR = AC/PR
Then, △ABC∼△PQR
where the symbol ∼ is read as ‘is similar to’.
Conversely
If △ABC is similar to △PQR, then
∠A = ∠P, ∠B = ∠Q, ∠C = ∠R and,
AB/PQ = BC/QR = AC/PR

Note: The ratio of any two corresponding sides in two equiangular triangles is always the same.

Theorem 1 ( Thales theorem)

If a line is drawn parallel to one side of a triangle to intersect the other two sides in distinct points, then other two sides are divided in the same ratio.
This theorem is known as the Basic Proportionality Theorem (BPT) or Thales theorem.

Triangles Class 10 Notes Maths Chapter 6

Given: Triangles Class 10 Notes Maths Chapter 6

To Proof: Triangles Class 10 Notes Maths Chapter 6

Proof:

Now, Triangles Class 10 Notes Maths Chapter 6

Triangles Class 10 Notes Maths Chapter 6

Now,

Triangles Class 10 Notes Maths Chapter 6

 

Question for Chapter Notes: Triangles
Try yourself:
Which of the following conditions determine if two polygons are similar?
View Solution
                                                                                                  

Theorem 2 ( Converse of Thales theorem)

If a line divides any two sides of a triangle in the same ratio, then the line is parallel to the third side.


Triangles Class 10 Notes Maths Chapter 6

Given: In ΔABC, DE is a straight line such thatTriangles Class 10 Notes Maths Chapter 6

To prove DE || BC.

Construction: If DE is not parallel to BC, draw DF meeting AC at F.

Proof: In ΔABC, let DF || BC

[∴ A line drawn parallel to one side of a Δ divides the other two sides in the same ratio.]

Triangles Class 10 Notes Maths Chapter 6

⇒ FC = EC.

It is possible only when E and F coincide

Hence, DE || BC.

Example 1:  If a line intersects sides AB and AC of a triangle ABC at D and E respectively and is parallel to BC, 

prove that Triangles Class 10 Notes Maths Chapter 6

Triangles Class 10 Notes Maths Chapter 6

Solution: 

Triangles Class 10 Notes Maths Chapter 6

 If a line is drawn parallel to one side of a triangle to intersect the other two sides in distinct points, the other two sides are divided in the same ratio.

Triangles Class 10 Notes Maths Chapter 6

Criteria For Similarity of Triangles


Two triangles are said to be similar triangles if their corresponding angles are equal and their corresponding sides are proportional (i.e., the ratios between the lengths of corresponding sides are equal).
For example:
If in ∆ABC and ∆PQR
Triangles Class 10 Notes Maths Chapter 6∠A = ∠P, ∠B = ∠Q, ∠C = ∠R and,
AB/PQ = BC/QR = AC/PR
The, △ABC∼△PQR
where, symbol ∼ is read as, ‘is similar to’.
Conversely
If △ABC is similar to △PQR, then
∠A = ∠P, ∠B = ∠Q, ∠C = ∠R and,
AB/PQ = BC/QR = AC/PR

1. AAA Similarity

Theorem 3 : If in two triangles, if corresponding angles are equal, then their corresponding sides are in the same ratio i.e., they are proportional, and hence the two triangles are similar.

This criterion is referred to as the AAA (Angle–Angle–Angle) criterion of similarity of two triangles.

Triangles Class 10 Notes Maths Chapter 6

Triangles Class 10 Notes Maths Chapter 6

Triangles Class 10 Notes Maths Chapter 6

2. AA Similarity


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

Triangles Class 10 Notes Maths Chapter 6If ∠A=∠C and ∠B=∠D then ABCDEF

Example: In theΔABC length of the sides are given as AP = 5 cm , PB = 10 cm and BC = 20 cm. Also PQ||BC. Find PQ.

Triangles Class 10 Notes Maths Chapter 6

Solution: In ΔABC and ΔAPQ, ∠PAQ is common and ∠APQ = ∠ABC (corresponding angles)

⇒ ΔABC ~ ΔAPQ (AA criterion for similar triangles)

AP/AB = PQ/BC

⇒ 5/15 = PQ/20

⇒ PQ = 20/3 cm

3.  SSS similarity

If in two triangles, sides of one triangle are proportional to (i.e., in the same ratio of ) the sides of the other triangle, then their corresponding angles are equal and hence the two triangles are similiar

This criterion is referred to as the SSS (Side–Side–Side) similarity criterion for two triangles.

Triangles Class 10 Notes Maths Chapter 6

Triangles Class 10 Notes Maths Chapter 6

Triangles Class 10 Notes Maths Chapter 6

Triangles Class 10 Notes Maths Chapter 6

Triangles Class 10 Notes Maths Chapter 6

Using Theorem: if a line divides two sides of a triangle in the same ratio, then the line is parallel to the third side. This theorem is often referred to as the Basic Proportionality Theorem or Thales' Theorem.

Triangles Class 10 Notes Maths Chapter 6

Triangles Class 10 Notes Maths Chapter 6

Triangles Class 10 Notes Maths Chapter 6

Triangles Class 10 Notes Maths Chapter 6

Example: Two triangles ABC and DEF are similar such that AB = 8cm, BC = 10cm, CA =y cm, DE = 6 cm, EF = x cm and FD = 9 cm . Find the Values of x and y?

Solution:  As ABCDEF  then  AB/DE=AC/DF=BC/EF

So now putting values 8/6=y/9=10/x

8/6=y/9   and 8/6=10/x

4/3=y/9 and 4/3=10/x

4*9=y*3 and 4*x=10*3 (Cross multiplying)

so y comes out to be =12

and x=7.5

Triangles Class 10 Notes Maths Chapter 6

4.    SAS Similarity


Theorem:  If one angle of a triangle is equal to one angle of the other triangle and the sides including these angles are proportional, then the two triangles are similar.

This criterion is referred to as the SAS (Side–Angle–Side) similarity criterion for two triangles.

If AB/ED=BC/EF and ∠B=∠E Then ABCDEF

 Triangles Class 10 Notes Maths Chapter 6

Triangles Class 10 Notes Maths Chapter 6

Example: Determine if the following triangles are similar. If so, write the similarity criteria

Triangles Class 10 Notes Maths Chapter 6

Solution:

We can see that BF∠B=∠F and these are both included angles. We just have to check that the sides around the angles are proportional.

ABDFBCFE=128=32=2416=32BC/FE=24/16=3/2  and AB/DF=12/8=3/2

Since the ratios are the same ΔABCΔDFEΔABC∼ΔDFE by the SAS Similarity Theorem.

Question for Chapter Notes: Triangles
Try yourself:
In triangle ABC, if angle A is equal to angle C, and AB/BC = 3/5, which similarity criterion can be used to prove that triangle ABC is similar to triangle XYZ?
View Solution







Summary

  • Similar Figures: Figures with the same shape, regardless of size, are termed similar figures.
  • Congruence vs. Similarity: While all congruent figures are similar, the reverse is not necessarily true.
  • Conditions for Similar Polygons: Two polygons with the same number of sides are similar if their corresponding angles are equal, and their corresponding sides are in proportion.
  • Parallel Lines and Triangle Side Division: Drawing a line parallel to one side of a triangle divides the other two sides in the same ratio.
  • Converse of Triangle Side Division: If a line divides any two sides of a triangle in the same ratio, it is parallel to the third side.
  • AAA Similarity Criterion: If corresponding angles in two triangles are equal, their corresponding sides are in the same ratio, and the triangles are similar.
  • AA Similarity Criterion: If two angles in one triangle are respectively equal to two angles in another triangle, the triangles are similar.
  • SSS Similarity Criterion: If corresponding sides in two triangles are in the same ratio, their corresponding angles are equal, leading to similarity.
  • SAS Similarity Criterion: If one angle of a triangle is equal to one angle of another triangle, and the sides including these angles are in proportion, the triangles are similar.

The document Triangles Class 10 Notes Maths Chapter 6 is a part of the Class 10 Course Mathematics (Maths) Class 10.
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FAQs on Triangles Class 10 Notes Maths Chapter 6

1. What are similar figures?
Ans. Similar figures are shapes that have the same shape but are different in size. They have the same angles and proportional sides.
2. How can you determine if two triangles are similar?
Ans. Two triangles are similar if their corresponding angles are congruent and their corresponding sides are in proportion. This can be determined using the Angle-Angle (AA) similarity criterion or the Side-Angle-Side (SAS) similarity criterion.
3. What are the criteria for similarity of triangles?
Ans. The criteria for similarity of triangles are Angle-Angle (AA), Side-Angle-Side (SAS), and Side-Side-Side (SSS). These criteria determine if two triangles are similar based on their angles and sides.
4. Can triangles with different side lengths be similar?
Ans. Yes, triangles with different side lengths can still be similar if their corresponding angles are congruent and their corresponding sides are in proportion.
5. How is similarity of triangles useful in real life applications?
Ans. Similarity of triangles is useful in various real-life applications such as architecture, engineering, and map-making. It helps in determining heights of buildings, calculating distances on maps, and designing structures that are proportional to each other.
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