Suppose you are given a square.  Of course it is easy to determine its area by taking square of the length of its side.  However, it is much harder to find the area of an irregular shape.[br][br]Here we consider any polygon and try to find its area.

Euclid's idea is very simple - construct a square whose area equals the area of this polygon.[br][br]But how to do it? We can first cut the polygon into triangles.

Then for each of these triangles, we can construct a parallelogram with a fixed base length and angle such that it has the same area as that of the triangle.  Stacking up all these parallelogram to form a big parallelogram.

The last step is to construct a square whose area equals the area of the big parallelogram.