Question 151902
Remove the parenthesis and re-group the terms.
ab(x^2+y^2)- xy(a^2+b^2)
= abx^2 + aby^2 - a^2xy - b^2xy
= abx^2 - a^2xy - b^2xy + aby^2
= (abx^2 - a^2xy) - (b^2xy - aby^2)
= ax(bx - ay) - by(bx - ay)
= (bx - ay)(ax - by)