Question

how do you find the sum of the first 12 terms of the sequence.

1, -4, -9, -14, . . .

Answer 1

Is it arithmetic or geometric?

Answer 2

@Mertsj it doesn't say :\

Answer 3

I know. You have to figure it out.

Answer 4

Do you always add the same number or multiply by the same number to get the next term.

Answer 5

@Mertsj you add -5 so it's arithmetic?

Answer 6

yes. and you have identified the common difference. So now hunt up your formula for the sum of an arithmetic sequence and plug into it.

Answer 7

First you need to find the 12th term. Can you do that?

Answer 8

\[a _{n}=a _{1}+(n-1)d\]

Answer 9

@Mertsj -53 is the 12th term

Answer 10

Use that to find the 12th term.

Answer 11

I got -54

Answer 12

\[a _{12}=1+(12-1)(-5)=1-55=-54\]

Answer 13

\[S _{12}=\frac{12(1-54)}{2}\]

Answer 14

@Mertsj sorry thats what i was supposed to put up. do i plug -54 into Sn=n(a1+an)/2 as an?

Answer 15

yes

Answer 16

@Mertsj hen it's 318?

Answer 17

I got -318

Answer 18

thank you!!!! @Mertsj

Answer 19

yw