x^2+y^2=(x+y)^2-2xy
即7=3^2-2xy
则2xy=9-7=2
xy=1
x^3+y^3=(x+y)(x^2-xy+y^2)
=3×(7-1)
=18
(x^2+y^2)(x^3+y^3)
=x^5+x^2y^3+y^2x^3+y^5
将xy=1代入上式
得x^5+y^5+y+x
=x^5+y^5+3
又(x^2+y^2)(x^3+y^3)
=7×18
=126
即
x^5+y^5+3=126
则
x^5+y^5=123
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