Đáp án:
Đặt `A = 1/(1.2.3) + 1/(2.3.4) + ............... + 1/(2015.2016.2017)`
`2A = 2/(1.2.3) +2/(2.3.4) + ............... + 2/(2015.2016.2017)`
`2A = 1/(1.2) - 1/(2.3) + 1/(2.3) -1/(3.4) + ................. +1/(2015.2016) - 1/(2016.2017)`
`2A =1/2 - 1/(2016.1027)`
` A = (1/2 - 1/(2016.2017) ) : 2`
` A = 1/4 - 1/(2.2016.2017)`
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