PARTIAL SUMS OF THE GEOMETRIC SERIES

Sn = a + aq + aq2 + … + aqn

MULTIPLY BOTH SIDES BY q

qSn = aq + aq2 + … + aqn+1

SUBTRACT TERM BY TERM

Sn - qSn = a - aqn+1

Sn = a (1 - qn+1)/(1 - q) if q ? 1

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