Question Number 10369 by Tawakalitu ayo mi last updated on 05/Feb/17

Find the sum of the series  2 + 5 + 8 + 12 ... n

Commented bymrW1 last updated on 06/Feb/17

the n−th term of the series is not  clear. please check.

Answered by arge last updated on 06/Feb/17

d=3  a_1 =2  a_n =?    a_n =a_1 +(n−1)d    a_n =2+3n−3    a_n =3n−1    a_n =((2S_n )/n) −a_1       3n^2 −n=2S_n −a_1 n    3n^2 +n=2S_n     S_n =((n(3n+1))/2)