Area of Similar Triangles


 
 
Concept Explanation
 

Area of Similar Triangles

Theorem: Ratio of the areas of two similar triangles is equal to the ratio of the square of their corresponding sides.

Given:  large Delta ABC sim Delta PQR

To Prove:

large frac{area(Delta ABC)}{area(Delta PQR)}=frac{AB^2}{PQ^2}=frac{BC^2}{QR^2}=frac{AC^2}{PR^2}

Construction: Draw AD large perp BC and PS large perp QR

Proof: In large Delta ABD and large Delta PQS

large angle B = angle Q                      large [;Delta ABC sim Delta PQR;]

large angle D = angle S                       large [ ;Each ;90^0;]

large Rightarrow Delta ABD sim Delta PQS                     [ By AA Similarity Criterion ]

Ratio of corresponding sides of similar triangle

large frac{AB}{PQ}=frac{AD}{PS}                     ..........................(1)

But we are given that large Delta ABC sim Delta PQR

    large frac{AB}{PQ}=frac{AC}{PR}=frac{BC}{QR}                 ..........................(2)

From Eq (1) and (2)

large frac{BC}{QR}=frac{AD}{PS}

Now

large frac{area(Delta ABC)}{area(Delta PQR)}=frac{frac{1}{2}X BC;X;AD}{frac{1}{2}X QR;X;PS}            large [;area ;of ;Delta = frac{1}{2};X;base;X;height;]

large frac{area(Delta ABC)}{area(Delta PQR)}=frac{BC;X;AD}{QR;X;PS}=frac{BC^2}{QR^2}

As large frac{AB}{PQ}=frac{AC}{PR}=frac{BC}{QR}

Hence  large frac{area(Delta ABC)}{area(Delta PQR)}=frac{AB^2}{PQ^2}=frac{BC^2}{QR^2}=frac{AC^2}{PR^2}

Illustration: If the area of two similar triangles are equal, prove that they are congruent.

Let large Delta ABC sim Delta PQR

large Rightarrow large frac{AB}{PQ}=frac{AC}{PR}=frac{BC}{QR}

Given      

large frac{area(Delta ABC)}{area(Delta PQR)}=1

We know that

large frac{area(Delta ABC)}{area(Delta PQR)}=frac{AB^2}{PQ^2}=frac{BC^2}{QR^2}=frac{AC^2}{PR^2}

large Rightarrow frac{AB^2}{PQ^2}=frac{BC^2}{QR^2}=frac{AC^2}{PR^2}=1

large Rightarrow AB = PQ , BC= QR and AC = PR

Hence

large Delta ABC cong Delta PQR     [By SSS Criterion of Congruence]

Sample Questions
(More Questions for each concept available in Login)
Question : 1

In the following figure, bigtriangleup ABCsim bigtriangleup bigtriangleup DEF. If AB = 2DE and area of bigtriangleup ABC=56;cm^{2}, find the area of bigtriangleup DEF.

Right Option : C
View Explanation
Explanation
Question : 2

large Delta AMBsim Delta CMD. Also 2ar (large Delta AMB) = ar (large Delta CMD) the length of MD is ___________________

Right Option : A
View Explanation
Explanation
Question : 3

Two triangles ABC and DEF are similar to each other in which AB = 10 cm and DE = 8 cm. Find the ratio of the areas of bigtriangleup ABC;and;bigtriangleup DEF.

Right Option : A
View Explanation
Explanation
 
Video Link - Have a look !!!
 
Language - English
 
 
 
Related Videos
Language - English



Students / Parents Reviews [10]