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How to get the formula for Sn of an arithmetic series?

Tn = a+(n -1)d

sn = n/2(a + n)

4. For the series 3 + 9 + 15 + 21... Find:
a) an expression for tn
b) an expression for sn
c) the answer if s = 20
d) the answer if t = 115

5. Find the # of terms in the sequencece 2,6,10... 626

I need a step by step procedure for solving these problems.

Solution Preview

4. In this series a=3, d=6. So
<br> a) tn=3+(n-1).6=6n-3
<br> b) sn=n(3+n)/2
<br> c) if s=20, then n(n+3)/2=20, ...

Solution Summary

Solution contains a detailed explanation on how to find the general formula for Sn of an arithmetic series.