Réponse :

[(x²+2xy+y²)+(x²-2xy+y²)]/2 =

(x²+2xy+y²+x²-2xy+y²)/2=

(2x²+2y²)/2 =x²+y²

[(x+y)²-(x-y)²]/4=

[(x²+2xy+y²)-(x²-2xy+y²)/4=

(x²+2xy+y²-x²+2xy-y²)/4=

(2xy+2xy)/4= 4xy/4 =xy

Explications étape par étape