Posted by **Crystal** on Saturday, January 12, 2013 at 4:57pm.

Is there a way to find the term number of a sum in an arithmetic series? Like if you had the sum of the first 12 terms in a series could you find the term value of term 12 using the sum?

For example in one of my previous questions :

For each series, calculate t12 and S12

a) 37 + 41 + 45 + 49 + ...

I calculated S12 to be 708. Using 708 is there a way to find t12?

- Math -
**Reiny**, Saturday, January 12, 2013 at 5:39pm
There are two common formulas for the sum of n terms of an AS

sum(n) = (n/2)(2a + (n-1)d)

and

sum(n) - (n/2)(first term + last term)

if you know S12 = 708, and we know a = 37 and n=12

708 = (12/2)(37 + last)

708 = 6(37+last)

118 = 37 + last

last = 81

but the last term would be term(12)

so term12 = 81

- Math -
**Crystal**, Saturday, January 12, 2013 at 5:43pm
Thank you!!

- Math -
**Reiny**, Saturday, January 12, 2013 at 5:44pm
welcome

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