Question

calculati :
S=$\frac{1}{1 X 2}+ \frac{1}{2 X 3} + \frac{1}{3 X4} +.........+ \frac{1}{89X90} =$

Answer 1

S = 1/1 - 1/2 + 1/2 - 1/3 +1/3 - 1/4 +........+1/89 - 1/90 = 1 - 1/90 = 89 / 90