Question

已知x+y=7,xy=10.求(1)2(x²+y²)的值;(2)(x-y)^4

Answer 1

1）2(x²+y²)=2[(x+y)^2-2xy]=2*(49-20)=58

2)(x-y)^2=(x+y)^2-4xy=49-40=9

(x-y)^4=[(x-y)^2]^2=9^2=81