Arithmetic series question

[SarahWhite]

I can seem to figure out the following problem:

In an arithmetic series, t[sub:2dualm3v]21[/sub:2dualm3v] is 4 times the value of t[sub:2dualm3v]11[/sub:2dualm3v] and S[sub:2dualm3v]10[/sub:2dualm3v] = -65. Find t[sub:2dualm3v]3[/sub:2dualm3v].

I don't know how to approach this question at all. If anyone has any tips on how to go about this problem, I'd be so grateful. I have formulas if they're needed.

 

[tkhunny]

You should have a general formula.

\(\displaystyle t_{0} = a_{0}\)
\(\displaystyle t_{1} = a_{0} + d\)
\(\displaystyle t_{2} = a_{0} + 2d\)
etc...

Then,

\(\displaystyle t_{11} = a_{0} + 11d\)
\(\displaystyle t_{21} = a_{0} + 21d\)

You tell me the formula for \(\displaystyle s_{n}\)

Note: I started numbering with zero (0). If you text or instructor starts numbering with one (1), things will be a little different. You'll have to assess that.