|x+y|^2+|x-y|^2
= (x+y,x+y) + (x-y,x-y)
= (x,x)+2(x,y)+(y,y) + (x,x)-2(x,y)+(y,y)
= 2(x,x)+2(y,y)
= 2|x|^2+2|y|^2.