If x2+y2=xy, the (x+y)4=
A
xy
B
x2y2
C
9x2y2
D
(x2+y2)2
Correct Answer: 9x2y2
x2 + y2 = xy ( x + y) 2 - 2xy = xy ( x + y) 2 = 3xy ( x + y) 4 = 9x2y2