Posted by **Rebekah** on Sunday, May 12, 2013 at 5:31pm.

Find a formula for the nth term if...

a)the 8th term is 8 and the 20th term is 44

b) the 8th term is 4 and the 18th term is -96

thanks!! I am just confused how you do this if you don't know what n is.

- Precalc -
**Steve**, Monday, May 13, 2013 at 12:04am
I assume these are arithmetic sequences, so

a)

Tn = a+(n-1)d so

a+7d = 8

a+19d = 44

12d = 36, so d=3

a = -13

or, you can do it like this.

T20-T8 = 12d = 44-8 = 36, so d=3

similarly, for

b)

10d = -100

d = -10

a = 74

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