Ex.6.4 Q8 Triangles Solution - NCERT Maths Class 10

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\(ABC\) and \(BDE\) are two equilateral triangles such that \(D\) is the mid-point of \(BC.\) Ratio of the areas of triangles \(ABC\) and \(BDE\) is

(A) \(2 : 1\) (B) \(1 : 2\) (C) \(4 : 1\) (D) \(1 : 4\)

Text Solution



\(AAA\) criterion.


\(\Delta A B C \sim \Delta B D E\) \((\because \text{equilateral triangles)}\)

The ratio of the areas of two similar triangles is equal to the square of the ratio of their corresponding sides.

\[\begin{aligned} \frac{\text {Area } \Delta A B C}{\text {Area } \Delta B D E} &=\frac{(B C)^{2}}{(B D)^{2}} \\ &=\frac{(B C)^{2}}{\left(\frac{B C}{2}\right)^{2}}[ D \text { is the midpoint of } \mathrm{BC}] \\ &=\frac{(B C)^{2} \times 4}{(B C)^{2}}\\&=4 \end{aligned}\]

Area of \(\,\Delta ABE\) \(:\) Area of\(\,\Delta BDE\) \(= 4:1\)

The answer is (c) \( 4:1\)

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