tính nhanh: 1/1x2x3 + 1/2x3x4 + 1/3x4x5 + .... + 1/18x19x20 giúp mình với

`1/(1xx2xx3) + 1/(2xx3xx4) + 1/(3xx4xx5) + ... + 1/(18xx19xx20)`

`= 1/2 xx (2/(1xx2xx3) + 2/(2xx3xx4) + 2/(3xx4xx5) + ... + 2/(18xx19xx20))`

`= 1/2 xx (1/(1xx2) - 1/(2xx3) + 1/(2xx3) - 1/(3xx4) + 1/(3xx4) - 1/(4xx5) + ... + 1/(18xx19) - 1/(19xx20))`

`= 1/2 xx (1/(1xx2) - 1/(19xx20))`

`= 1/2 xx (1/2 - 1/380)`

`= 1/2 xx 189/380`

`= 189/760`