1/10×11+1/11×12+…… 1/19×21+1/20

1/10×11+1/11×12+…… 1/19×21+1/20

The original formula = (1 / 10-1 / 11) + (1 / 11-1 / 12) +. + (1 / 19-1 / 20) = 1 / 10-1 / 20 = 1 / 20