Question

3+10+17+24+...+7003+7010, How do I find the "n' for the formula Sn = n/2 (a1+an)?

Answer 1

well I'd use the equation for a term in an arithmetic sequence rather than the sum equation.

you know the nth term is 7010 and the 1st term is 3 as well as the common difference is 7

so substitute them into

\[a_{n}= a_{1} + (n - 1) \times d\]

which becomes

\[7010 = 3 + (n - 1) \times 7\]

now solve for n

Answer 2

Thanks!