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Asked by Jon

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
17 years ago

Answers

Answered by Reiny
YOur third line is wrong

should be:

189 = a(1 - 1/8)/(1 - 1/2)
189 = a(7/8)/(1/2)
189 = a(7/4)
756 = 7a
108 = a

(check 108+54+27 = 189 !)
17 years ago

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