1 / 1x2x3 + 1 / 2x3x4 + 1 / 3x4 x5 + 1 / 4x5x6 =

1 / 1x2x3 + 1 / 2x3x4 + 1 / 3x4 x5 + 1 / 4x5x6 =

= 1 / 2 × (1 / 1 × 2 - 1 / 2 × 3 + 1 / 2 × 3 - 1 / 3 × 4 + 1 / 3 × 4 - 1 / 4 × 5 + 1 / 4 × 5 - 1 / 5 × 6)
= 1 / 2 × (1 / 1 × 2 - 1 / 5 × 6)
= 7 / 30