解(1)：x²+y²=(x+y)²-2xy=a²-2b(2)：x³+y³=(x+y)(x²-xy+y²)=(x+y)[(x²+y²)-xy]=a[(a²-2b)-b)=a(a²-3b)(3)：x^4+y^4=(x²+y²)²-2x²...