[(x^2+y^2)-(x-y)^2+2y(x-y)]/4y =[(x^2+y^2)-(x^2-2xy+y^2)+2xy-2y^2]/4y =[x^2+y^2-x^2+2xy-y^2+2xy-2y^2]/4y =(4xy-2y^2)/4y =2y(2x-y)/4y =(2x-y)/2=10/2=5
