x+y=1 x^3+y^3=(x+y)(x^2-xy+y^2)=x^2+y^2-xy=(x+y)^2-3xy=1-3xy=2 xy=-1/3 (x+y)^2=x^2+y^2+2xy=1 x^2+y^2=1+2/3=5/3 x的平方分之一加y平方分之一=1/x^2+1/y^2=(x^2+y^2)/(xy)^2=15