Question

what is the sum of a 28-term arithmetic sequence where the first term is 12 and the last term is 225?

Answer 1

\[S_{n} = (a_{1} + a_{n}).n \div2\]\[a_{1} =12 \]\[a_{n} = a_{28} =255 \]n=28

Put it in the formula and you're good to go.