The area of a circle inscribed in an equilateral triangle is 154 cm2. Find the perimeter of the triangle.
Let the radius of the inscribed circle be r cm.
Given:
Area of the circle $=154 \mathrm{~cm}^{2}$
We know
Area of the circle $=\pi r^{2}$
$\Rightarrow 154=\frac{22}{7} r^{2}$
$\Rightarrow \frac{154 \times 7}{22}=r^{2}$
$\Rightarrow r^{2}=49$
$\Rightarrow r=7$
In a triangle, the centre of the inscribed circle is the point of intersection of the medians and altitudes of the triangle. The centroid divides the median of a triangle in the ratio 2:1.
Here,
AO:OD = 2:1
Now,
Let the altitude be h cm.
We have:
$\angle A D B=90^{\circ}$
$O D=\frac{1}{3} A D$
$O D=\frac{h}{3}$
$\Rightarrow h=3 r$
$\Rightarrow h=21$
Let each side of the triangle be a cm.
In the right - angled $\triangle \mathrm{ADB}$, we have :
$A B^{2}=A D^{2}+D B^{2}$
$a^{2}=h^{2}+\left(\frac{a}{2}\right)^{2}$
$4 a^{2}=4 h^{2}+a^{2}$
$3 a^{2}=4 h^{2}$
$a^{2}=\frac{4 h^{2}}{3}$
$a=\frac{2 h}{\sqrt{3}}$
$a=\frac{42}{\sqrt{3}}$
∴ Perimeter of the triangle = 3a
$=3 \times \frac{42}{\sqrt{3}}$
$=\sqrt{3} \times 42$
$=72.66 \mathrm{~cm}$