I'm unclear on how to solve this problem, can anyone help me please? (:

"Given the sequence -3,0,3, which of the following is the equivalent to

Question

I'm unclear on how to solve this problem, can anyone help me please? (:
 
"Given the sequence -3,0,3, which of the following is the equivalent to

anonymous · Answer

$$\sum_{n=5}^{8} a _{n}$$

anonymous · Answer

54
102
144
162 

These are my choices.

unklerhaukus · Answer

$$\sum_{n=5}^{8} a _{n}=a_5+a_6+a_7+a_8$$

unklerhaukus · Answer

what is $-3,0,3,$   ?   

are they  $a_1, a_2,a_3$   ?

anonymous · Answer

Im not really sure, I think you're supposed to derive a "summation" from those. When I did that I got 6+3n but I'm not sure if that's right

anonymous · Answer

I got that from substituting 9 as the first term and 3 as i in the equation
$$a_{n}$$ = a1 + (n-1)d

unklerhaukus · Answer

how do you know 9 is the first term?

anonymous · Answer

Because n=5 so that means that I start on the first term of the original sequence?
so -3,0,3,6,9,12,...
so 9 is the 5th term, so I started there. From there I continued
9,12,15,18,21,24,27,30,33... and used 30 as the 8th term. 
I just sorta guessed on that part...

unklerhaukus · Answer

oh right, so $$a_5=9,\quad a_6=12, \quad a_7=15, \quad a_8=18$$

anonymous · Answer

Okay, so if those are the values, then what would you do afterwards?

unklerhaukus · Answer

just add em

anonymous · Answer

oh, wow okay. So I guess 9 wasn't the first term haha

unklerhaukus · Answer

$$\sum_{n=5}^{8} a _{n}=9+12+15+18=$$

anonymous · Answer

I got 54 correct?

unklerhaukus · Answer

i think thats right

anonymous · Answer

Okay, thank you so much! (: