a*b+(b-a)*b=ab+a-b+(b-a)b+b-a-b=ab+a-b+b²-ab+b-a-b=b²-b;
a*b+(b-a)*b=ab+a-b+(b-a)b+b-a-b=ab+a-b+b^2-ab+b-a-b=b^2-b
