1 × 2 / 1 + 2 × 3 / 1 + 3 × 4 / 1 + 4 × 5 / 1 +... + 39 × 40 / 1

1 × 2 / 1 + 2 × 3 / 1 + 3 × 4 / 1 + 4 × 5 / 1 +... + 39 × 40 / 1

1 × 2 / 1 + 2 × 3 / 1 + 3 × 4 / 1 + 4 × 5 / 1 +... + 39 × 40 / 1
=1-1/2+1/2-1/3+1/3-1/4+…… +1/39-1/40
=1-1/40
=39/40