1 / 10 × 11 + 1 / 11 × 12 +...1 / 19 × 21 + 1 / 20

1 / 10 × 11 + 1 / 11 × 12 +...1 / 19 × 21 + 1 / 20

원 격 = (1 / 10 - 1 / 11) + (1 / 11 - 1 / 12) + + + + (1 / 19 - 1 / 20) = 1 / 10 - 1 / 20 = 1 / 20