1/(1*2) + 1/(2*3) + 1/(3*4) + ...+ 1/(49*50)
= 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + ...+ 1/49 - 1/50
= 1/1 - 1/50
= 49/50