The formula to find the area of a tri-angle is ....

The formula to find area of a rectangle is :

The formula to find area of equilateral triangle is :

The formula to find area of a square is :

Why is the formula for finding the area of a rectangle related to the formula for finding the area of parallelogram?

The formula for finding area of square is

Write the formula to find area of a sector of a circle, if angle at the centre is 'theta' degrees.