Đáp án:
`N = 1/(1 . 2 . 3) + 1/(2 . 3 . 4) + 1/(3 . 4 . 5) + ................. +1/( a. (a+1). (a+2) )`
`N =1/2 .(2/(1.2.3) + 2/(2.3.4) + 2/(3.4.5) + ........... + 2/(a . (a+1) . (a+2) ) )`
`N =1/2 . ( 1/(1.2) - 1/(2.3) + 1/(2.3) - 1/(3.4) + .......... + 1/(a . (a+1)) - 1/(( a+1) . (a+2)) )`
`N = 1/2 . (1/(1.2) - 1/((a+1).(a+2)) )`
`N = 1/4 - 1/( 2. (a+1). (a+2) )`
`text{ @toanisthebest}`