Question

Find the area of an isoscales right triangle, if one of the equal sides is 20 cm long

Answer 1

Area = base x height x 1/2 = 20 x 20 x 1/2 = 200 cm²

Answer 2

ANSWER

In the isosceles right triangle, the hypotenuse is the longest side of the triangle. It is not equal to any other side of the triangle. 
So, the legs of the triangle are the equal sides, which are perpendicular to each other, also forming the base and height of the triangle. 
The measure of each of the side is 20 cm. 
So, the area of the triangle
= $\frac{1}{2}$ × Base × Height
= $\frac{1}{2}*20*20$
= 200 cm²

∴ The area of the triangle is 200 cm². 

Hope this may help you. 

If you have any doubt, then you can ask me in the comments.