Đặt:
`A = 1/(1 × 2 × 3) + 1/(2 × 3 × 4) + 1/(3 × 4 × 5) + ... + 1/(10 × 11 × 12)`
`2A = 2 × (1/(1 × 2 × 3) + 1/(2 × 3 × 4) + 1/(3 × 4 × 5) + ... + 1/(10 × 11 × 12))`
`2A = 1/(1 × 2) × 1/(3 × 4) + 1/(3 × 4 × 5) + ... + 1/(10 × 11 × 12)`
`2A = 1/2 - 1/(2 × 3) + 1/(3 × 4) - 1/(4 × 5) + ... + 1/(10 × 11) - 1/(11 × 12)`
`2A = 1/2 - 1/(11 × 12)`
`2A = 1/2 - 1/132`
`2A = 65/132`
`A = 65/132 : 1/2`
`A = 65/264`
