Question

find the sum of the first 50 terms of the arithmetic sequence -15,-9,-3,3....

Answer 1

You will need the formula: \[\sum_{k=1}^{n}k = \frac{1}{2}n(n+1)\]
Write down the sequence in terms of an index, then put it in a sum and simplify.

Answer 2

i cant get it

Answer 3

Let's say you take as index n=1,2,3,... so your sequence is
\(a_1 = -15\)
\(a_2 = -9\)
\(a_3 = -3\)
...
so what is a general term for a_n ?