How to Calculate the area of various polygons

We can calculate the area of the simple polygons like square, rectangle, triangle, pretty easily. Area of square can be calculated by drawing it on a graph paper, of unit cm length and breadth. Area of a rectangle is length X breadth. Area of a triangle is 1/2bh, half of product of base and height. For a parallelogram , area is the product of base and height. For a trapezoid , it is 1/2(b1+b2)h. For some other extreme polygons, we can calculate the area by making it different simple polygons like square or triangle. Then add the areas of triangle and square.