Répondre :
(x+y)²-(x-y)²
= x²+2*x*y+y²-x²-2*x*y+y²
= 2xy+2y²-2xy
= 2y²
10 001²-9 999²
= ( 10 001 - 2 )² - ( 9 999² + 2 )
= x²+2*x*y+y²-x²-2*x*y+y²
= 2xy+2y²-2xy
= 2y²
10 001²-9 999²
= ( 10 001 - 2 )² - ( 9 999² + 2 )