Question

What is the sum of the first 15 terms of the equation an=10n+21.
The answer is NOT 1374.

Answer 1

\(\bf S_{\color{red}{ n}}=\cfrac{{\color{red}{ n}}(a_1+a_{\color{red}{ n}})}{2}\qquad \qquad S_{\color{red}{ 15}}=\cfrac{{\color{red}{ 15}}(a_1+a_{\color{red}{ 15}})}{2}\)

Answer 2

Would the beginning of the sequence look like 1,31 ...
or 31, 41 ...?

Answer 3

Like what is a1 supposed to be?

Answer 4

just put n=1 in the An formula, u get a1=10*1+21=31
then put n=15 in the An forumla, u get a15=10*15+21
then u use Sn=(a1+a15)*15*0.5

Answer 5

= 1515

Answer 6

if you want to know the beggining of the formula
or I gather you mean the 1st term of the sequence
just set n = 1

Answer 7

\(\bf a_n=10n+21\) is an arithmetic sequence, it's just already combined including the common difference and "n" and 1st term, in a simplified version

Answer 8

ok thanks guys!