x+y=10 (x+y)^2=100 x^3+y^3=(x+y)(x^2-xy+y^2)=100 即10*{(x+y)^2-3xy}=100 10*(100-3xy)=100 所以xy=30 (x+y)^2=x^2+y^2+2xy=100 x^2+y^2=40
