The arithmetic series is 204, 210, 216, ….., 294
First term, a = 204
difference, D = 210 – 204 = 6
Last term, an = 294
By the formula of nth term of Arithmetic progression, we have;
an = 294
a+(n−1)d=294
204+(n−1)6=294
204+6n−6=294
198+6n=294
6n=294−198=96
Therefore, n=16
Thus, there are 16 terms in A.P., which we need to add.
Sn = n/2 [a + an]
S16 = 16/2 [204 + 294]
= 8 [498]
= 3984