## anonymous one year ago Sigma Notation

1. anonymous

$\sum_{n=4}^{15} 4(-3n)$

2. anonymous

$\sum_{n=1}^{15} 4(-3n)$

3. anonymous

|dw:1439769637735:dw|

4. anonymous

|dw:1439769669471:dw|

5. anonymous

@amistre64

6. amistre64

7. anonymous

I was thinking A.

8. amistre64

so, why are you thinking A? i need to know your thought process here in order to correct it

9. anonymous

Because it looks like an arithmetic sequence and i know that it uses the formula an=a1(n-1)d

10. amistre64

hmm, if it was arithmetic, then the difference between terms is constant 4 + (-16) = -12 -12 + (-16) .. is not 36, is it?

11. anonymous

No, you're right :)

12. amistre64

if its geometric, the common element is multiplied 4(-3) = -12 -12(-3) = 36

13. anonymous

yes. I wasn't looking at it like that, but I see your point now.

14. amistre64

we also know our index goes from 4 to 15 so that should help eliminate some options as well

15. amistre64

but i think you have some of your options displayed incorrectly ...

16. anonymous

What do you mean?

17. amistre64

|dw:1439770483912:dw|

18. anonymous

It means $\sum_{n=4}^{15}$

19. anonymous

then 4(-3)^n-1

20. amistre64

\Large helps magnify the test

21. anonymous

??

22. amistre64

\Huge is the zoomiest $\Huge \sum_{n=start}^{end}$

23. anonymous

ok :) Do you get what I wrote down now?

24. amistre64

we want to start at 4 and end at 15 right? ... but the only real option we have i believe you misprinted .. |dw:1439770648146:dw|

25. amistre64

with a geometric sequence, the series is represented as an exponent on the common ratio $\Huge \sum_{n=start}^{end}a_1(r)^{n-1}$

26. amistre64

agreed? if so, what is our solution?

27. anonymous

That was how it was written on my math packet so idk

28. anonymous

29. amistre64

hmm, then somewhere there is a misprint. C is 'closest' we have to the right format; but we would want it to start at 4, not 1 $\Huge \sum_{n=\color{red} 4}^{15}4(-3)^{n-1}$

30. anonymous

Ok isn't D saying that then?

31. anonymous

the summation of 4 times negative 3 to the n minus 1 power, from n equals 4 to 15.

32. amistre64

no, -4 is not 4 on the a_1 part

33. amistre64

|dw:1439770907174:dw|

34. anonymous

Oh! My mistake, there was no negative 4!

35. amistre64

in that case, its fine lol

36. anonymous

|dw:1439770953367:dw| Is this the answer then?

37. amistre64

as long as thats not a negative four .. id agree :)

38. anonymous

Thanks so much :)

39. amistre64

yw

40. anonymous

I have another, so I'll just post it in the open question section. K?

41. amistre64

k