If ${\cos }^{-1}x-{\cos }^{-1}\frac{y}{2}=\alpha$ where -1$\le$ x $\le$ 1, -2 $\le$ y $\le$ 2, $x⩽\frac{y}{2}$ , then for all x, y, 4x² – 4xy cos$\alpha$ + y² is equal to :