x(x+1)分之1加(x+1)(x+2)分之1加（x+2)(x+3)分之1加（x+3)(x+4)分之1等于多少?
问题描述：
x(x+1)分之1加(x+1)(x+2)分之1加（x+2)(x+3)分之1加（x+3)(x+4)分之1等于多少?
答
1/[x(x+1)] + 1/[(x+1)(x+2)] + 1/[（x+2)(x+3)] + 1/[（x+3)(x+4)] = 1/x - 1/(x+1) + 1/(x+1) - 1/(x+2) + 1/(x+2) - 1/(x+3) + 1/(x+3) - 1/(x+4) = 1/x - 1/(x+4) = 4/[x(x+4)]