Geometric progression
Submitted by Don Pilas on September 20, 2019 - 5:25pm
Find the sum of the first 5 terms of the geometric progression if the third term is 144 and the sixth term is 456.?
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Find the sum of the first 5 terms of the geometric progression if the third term is 144 and the sixth term is 456.?
The formula for the sum of
The formula for the sum of the first n terms of geometric progression is
$S_n = \dfrac{a_1(1 - r^n)}{1 - r}$
All we need to do is to find the first term a1 and the common ratio r.
To find the common ratio use the formula
$a_n = a_m r^{n - m}$
$a_6 = a_3 r^{6 - 3}$
With r known, you can solve for a1 using the formula:
$a_n = a_1 r^{n - 1}$
$a_6 = a_1 r^{6 - 1}$
With your a1 and r known, you can now calculate the sum S5.
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