cant find the ratio Watch

KloppOClock
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Geometric series has a=4 and r of 0<r<1.
The first, second and fourth term of this series form an arithmetic series.
Find the sum to infinity.

Here is what I've done so far....

4-4r=4r-4r^3
r^3-2r+1=0
r= 0,1,-1.

But all of these ratios don't fit the initial constraint.

Can I have some hints please (no solutions)
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KloppOClock
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never mind, managed to do it.

how do i delete my post?
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