如图所示,只要次数低于n,则其n阶导数就是0
φ(x) = f(x) + k(x-a1)(x-a2)......(x-an)= f(x) + k[x^n-(a1+a2+...+an)x^(n-1) +...... +(-1)^n a1a2...an]φ^(n) (x) = f^(n) (x) + kn!