The sum of an arithmetic series is 1356 and the first term is -1. If the common difference is 5, how many terms are in the sequence?

Question

ash2326 · Answer

@katiebugg Sum of n terms is given 1356
First term $a_1=-1$ and common difference=5
we know that sum to n terms is given as
$$S_n=\frac{n}{2}(a_1+a_n)$$
$a_n$ is the last term
Do you get till this?

anonymous · Answer

yess

ash2326 · Answer

we know $a_n$ is given as
$$a_n=a_1+(n-1)d$$
d=common difference
n=no. of terms
so
sum will become
$$S_n=\frac n2 (a_1+a_1+(n-1)d)$$
now pluigin the numbers here and post what do you get, will you?