S = 1-1/2 + 1/3-1/4 + 1/5-1/6 + ..... 1/499-1/500
= (1 + 1/3 + 1/5 + ..+ 1/499) - (1/2 + 1/4 + 1/6 + ...+ 1/500) - (1/2 + 1/4 + 1/6 + ...+ 1/500) + (1/2 + 1/4 + 1/6 + ...+ 1/500)
S = (1 + 1/2 + 1/3 + 1/4 + ....+ 1/500) - 2.(1/2 + 1/4 + 1/6 + ...+ 1/500)
= (1 + 1/2 + 1/3 + 1/4 + ....+ 1/500)- (1 + 1/2 + 1/3 + ...+1/250)
= 1/251 + 1/252 + ...+ 1/500.
Vậy S = 1/251 + 1/252 + ...+ 1/500