If x + 2y = 4 and xy = 18, what is the value of x^2 + 4y^2?
x + 2y = 4 square both sides ⇒ x^2 + 4xy + 4y^2 = 16 (1)
xy = 18 multiply through by 4 ⇒ 4xy = 72 (2)
Sub (2) into (1)
x^2 + 72 + 4y^2 = 16 subtract 72 from both sides
x^2 + 4y^2 = -56
