Question

what is the sum of the first fifteen terms in the sequence an = 10n + 21?

Answer 1

this is an arithmetic sequence

with a=31
and d=10

sum of all 15 terms of an arithmetic sequence would be S15=....

Answer 2

are u getting it?

\[Sn=\frac{ n }{ 2 }(2a+(n-1)d)\]

Answer 3

then subst a,d and n=15