Question

Find the number of terms, n, of the arithmetic series given a1=21, an=39, and sn=210.

Answer 1

`a_(1) = 21 \\ a_(n) = 39 \\S_(n) = (n)/(2) (a_(1) + a_(n))`

`210 = (n)/(2) (21 + 39) \\ 210 = (n)/(2) (60) \\ 210 = 30n \\ n = (210)/(30) = 7 \: terms`

Common difference is 3