,xnw(a,b)(nWN)有f(xi +x2+HI+xn)证:当n =1时,命题显然成立。1假设n =k时命题成乂,当n = k+1时,令A =(x1+x2+…+xk+xk中)k 1则a___又令B___2k2kC=XkJ(:-1)A' f(A尸"亭)弓[f(B)十f(C)]「X1 x2IMXkXk1k-1 A2kk1一1一 一 ....