Question

find the area of a isosceles triangle whose base is 30 centimetres

Answer 1

If one side is 30cm another one will also be 30cm. (Isosceles triangle)

It is a right angled triangle so the two equal sides must be base and perpendicular.

Since it is isosceles, the height is 30 cm. 
area = (½)(base)(height) = 900/2 = 450 cm²

The area of a isosceles triangle= 450 cm²