Determine the number of terms n

a1=19, an=96, Sn=690

Im a bit stuck on what to do here?

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- Nov 10th 2010, 06:28 AMjlaingArithmetic series
Determine the number of terms n

a1=19, an=96, Sn=690

Im a bit stuck on what to do here? - Nov 10th 2010, 06:31 AMharish21
$\displaystyle S_n=\dfrac{n}{2}(a_1+a_n)$