It's area is (a+b)(a+b) This area has 4 parts: The lower left b x b square.
So (a+b)^2 = a^ + 2ab + a^2 
It's area is (ba)^2. The animation below shows how this is the same as the illustration to the right. 
To show the rectangles are negative, they're colored black 
After it's over,
So (ba)^2 = a^2 2ab + b^2 
