Jika : [tex] x^{2} [/tex] = 8y + 79 dan [tex] y^{2} [/tex] = 8x + 79 Maka , nilai dari xy ?

bila x² = 8y +79
maka 79 = x²-8y

bila y² = 8x +79
naka 79 = y²-8x

dari dua oersamaan tersebut
x² -8y = y²-8x
x²-y² -8y +8x =0
(x-y) (x+y) + 8(x-y) = 0
(x-y) (x+y+8) = 0

x-y = 0
x = y

x+y+8=0
y+y+8=0
2y+8=0
y = -4
x = -4

maka xy = -4*-4 = 16