I know that if I have a set of numbers, let's say+
$1,2,3,4,5$
I can find the number of terms by subtraction the last term $5$ from the first terms $1$ and then add $1$: $(5-1)+1 = 5$, then the number of terms = 5.This is pretty much easy example just to make my question clearer.
*In arithmetic sequence I wanted to (know the number of terms) using these details:
$s_n = 270$
$d = 1$
$a_1 = 4$
I thought about it and used the formula: $s_n = {n\over 2}(2a(n-1)d)$
then I substitute
$270 = {n\over 2} (2(4)+(n-1)(1))$
$270 = {n\over 2}(8+n-1)$
$270 = {7n+n^2\over 2}$
$540 = 7n+n^2$ # I dont think this works ?
Best Answer
Solve the quadratic equation; and choose the positive value for n you obtain as solution.
Edit: