The area of a triangle is equal to the base times the height.
In a semi circle, the diameter is the base of the semi-circle.
This is equal to 2 x r (r = the radius)
If the triangle is an isosceles triangle with an angle of 45° at each end, then the height of the triangle is also a radius of the circle.
A = 1/2 x b x h formula for the area of a triangle becomes
A = 1/2 x 2 x r x r because:
The base of the triangle is equal to 2 x r
The height of the triangle is equal to r
A = 1/2 x 2 x r x r becomes:
A = r2