Question

(x-y)²- (x+y)² = -20. Determine quanto vale x.y

Answer 1

(x-y)²- (x+y)² = -20.

x²-2xy+y² -(x²+2xy+y²) = -20

x²-2xy+y² -x²-2xy-y² = -20

-4xy = -20

x.y = 5