## anonymous 5 years ago find the sum of the first 50 terms of the arithmetic sequence -15,-9,-3,3....

1. nowhereman

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.

2. anonymous

i cant get it

3. nowhereman

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 ?