Se aplica formula 1/n(n+1) = 1/n - 1/(n+1)
Ex : 1/1x2 = 1/1 - 1/2
1/2x3 = 1/2 - 1/3
S1 = 1/1x2 + 1/2x3 + 1/3x4 +........+ 1/89x90
S1 = 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 +........+1/88 - 1/89 + 1/89 - 1/90
S1 = 1/1 - 1/90 = (90-1)/90 = 89/90
