Question

S=n(a1+an)/2 gives the partial sum of an arithmetic sequence. What is the formula solved for an?

Answer 1

S = n(a1 + an)/2
2S = n(a1 + an)
2S = na1 + nan
nan = 2S - na1
an = (2S - n a1)/n

Answer 2

Just took the quiz.

The answer is an=2S-a1n/n