证:
设f(x)=a1(x-b2)(x-b3)+a2(X-b1)(x-b3)+a3(x-b1)(x-b2)
因为 f(b1)=a1(b1-b2)(b1-b3) ,f(b2)=a2(b2-b1)(b2-b3)
因为b1