Question

Determine o valor numérico de x² - y² + 5x + 5y, sabendo que x + y = 10 e x - y = 4.

Answer 1

$\sf{x^2-y^2+5x+5y}\\\sf{(x-y)\cdot(x+y)+5\cdot(x+y)=(x+y)\cdot(x-y+5)}\\\sf{10\cdot(4+5)=10\cdot9=90}$