write down the terms indicated in each of these arithmetic series.

$\displaystyle \frac{1}{4} + \frac{7}{8}........;12th, nth $

I worked out what the 12th term is, $\displaystyle 7\frac{1}{8} $ using this formulae $\displaystyle a + (n-1)d $

but I am not sure how to work out the nth term? do i use the same formulae?