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We know that:

The sum of the first four terms in the series: a + ar + ar^2 + ar^3
The sum of first two terms in the series: a + ar

How many times larger is the fourth term than the second term?

ar^3 = ar * X ?

The sum of the first four terms in the series is 5 times the sum of first two terms. So lets write that out

a + ar + ar^2 + ar^3 = 5 ( a + ar) expand the RHS

a + ar + ar^2 + ar^3 = 5a + 5ar subtract like terms

ar^2 + ar^3 = 4a + 4ar factor out on each side

ar^2 (1+ r) = 4a (1+r) divide (1+r) from each side

ar^2 = 4a divide by "a" on both sides

r^2 = 4

r = 2

Lets now plug this into the relation in question:

ar^3 = ar * X
a 2^3 = 2a * X
8a = 2a * X
4 = x

==> C
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Started with

a (r-1)^3 = 5a(r-1)

Ended with

r(r-2) = 4

Guessed that the r = 2 and with 2 spaces, it was 4

Definitely don't want to leave it to guessing always. Any way I could have done it better?
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Charli08
We know that:

The sum of the first four terms in the series: a + ar + ar^2 + ar^3
The sum of first two terms in the series: a + ar

How many times larger is the fourth term than the second term?

ar^3 = ar * X ?

The sum of the first four terms in the series is 5 times the sum of first two terms. So lets write that out

a + ar + ar^2 + ar^3 = 5 ( a + ar) expand the RHS

a + ar + ar^2 + ar^3 = 5a + 5ar subtract like terms

ar^2 + ar^3 = 4a + 4ar factor out on each side

ar^2 (1+ r) = 4a (1+r) divide (1+r) from each side

ar^2 = 4a divide by "a" on both sides

r^2 = 4

r = 2

Lets now plug this into the relation in question:

ar^3 = ar * X
a 2^3 = 2a * X
8a = 2a * X
4 = x

==> C
­Totally agree with teh approach here, but be careful at the end when you go from the step r^2 = 4 to r = 2. r could also equal negative two. Not a big deal in this problem as we only really need the value r^2, but could be an issue for other problems, always need to check the negative case. ­
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