Posted by **Jon** on Tuesday, March 4, 2008 at 12:18pm.

Find a1 in a geometric series for which Sn=189,r=1/2,and an=3.

Sn=a1(1-r^n)/1-r

189=a1(1-1/2^3)/1-1/2

189=a1(-.5)/.5

189/-.25

a1=-756

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