Math
posted by Jon .
Find a1 in a geometric series for which Sn=189,r=1/2,and an=3.
Sn=a1(1r^n)/1r
189=a1(11/2^3)/11/2
189=a1(.5)/.5
189/.25
a1=756

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 !)