An isosceles triangle has perimeter 30 cm and each of the equal sides is 12 cm. Find the area of the triangle.

We know that, 

The Heron's formula for calculating the area of the triangle,

Area of triangle = $\sqrt{s (s - a) (s - b) (s -c)}$

where, s is the semi-perimeter = half of the perimeter = 30/2 = 15 cm

and  a, b and c are the sides of the triangle 

Triangular sides are, a = 12 cm, b = 12 cm, c =?

Thus, Semi-perimeter 

s = (a $+$ b $+$ c)/2

15 = (12 $+$ 12 $+$ c)/2

15 = 24 $+$ c/2

30 =  24 $+$ c

c = 6 cm

Area of triangle = $\sqrt{15(15 - 12)(15 -12)(15 - 6)}$

= $\sqrt{15 \times 3 \times 3 \times 9}$

= $\sqrt{1215}$

= 9$\sqrt{15}$  cm²