你好哦~直接去括号，为：(x-y)(x-y)=x^2-xy-xy+y^2=x^2-2xy+y^2
(x-y)²=x²-2xy+y²
x-y=3(x-y)²=9x²+y²-2xy=9x²+y²-4=9x²+y²=5(x²+y²)²=25x^4+y^4+2x²y²=25x^4+y^4+8=25x^4+y^4=17
