In an arithmetic series, the terms of the series are equally spread out. For example, in

1 + 5 + 9 + 13 + 17, consecutive terms are 4 apart. If the first term in an arithmetic series is

3, the last term is 136, and the sum is 1,390, what are the first 3 terms?

Did I do this right?

$\displaystyle sn=n/2(a1+an)$

$\displaystyle 1390=n(139)/2$

$\displaystyle n=20$

$\displaystyle increment = (136-3)/(n-1)$

$\displaystyle 133/(20-1)$

$\displaystyle 133/19 = 7$

So it'd be $\displaystyle 3+10+17...136 = 1390.$