## anonymous one year ago Find the sum of the first 12 terms of the sequence. Show all work for full credit. 1, -4, -9, -14, . . . please this is my last question and i cant seem to get it right. if possible can someone work it out for me?

1. campbell_st

what do you know about the question..?

2. anonymous

i believe its an arithmetic sequence?

3. campbell_st

good start, what's the common difference..?

4. anonymous

ok that where i confuse myself

5. campbell_st

well what is the sequence changing by... common difference = 2nd term - 1st term check with 3rd term - 2nd term.... is it the same

6. anonymous

-5

7. campbell_st

great... and the 1st term a = 1 so for the sum of the sequence you can use the formula $S_{n} = \frac{n}{2}[2a + (n - 1) \times d]$ you are given a = 1, you found d = -5 and have been told n = 12 so substitute them and calculate the sum

8. anonymous

i get the plugging in but what does d equal? all the rest i plugged in

9. campbell_st

d is the common difference d = -5

10. anonymous

i see ok 1 min

11. anonymous

is it 6/-53

12. anonymous

that's what i got but I not entirely sure.

13. campbell_st

you are close the calculation becomes $S_{12} = \frac{12}{2} \times (-53)$

14. campbell_st

so then $S_{12} = 6 \times (-53)$

15. anonymous

so that is the final form?