Question

what should be subtract from x square plus y square minus 2xy to get x square plus y square

Answer 1

you should minus -2xy to get the desired value

Answer 2

Let's "K" is the number that should be sbutracted

(X^2 + Y^2 -2XY ) - (K) = X^2 + Y^2
=> -K = X^2+ Y^2 - X^2 - Y^2 + 2XY
=> K = -2XY