原式=1 - 1/1*2 - 1/2*3 - 1/3*4 -1/4*5 -````` -1/99*100
=1 -(1/1 - 1/2)-(1/2-1/3)-(1/3-1/4)-```````-(1/99-1/100)
=1 - 1/1 + 1/2 - 1/2 + 1/3 - 1/3 + 1/4-````````-1/99+1/100
=1/100