In: Advanced Math
Use the formula for the sum of the first n terms of a geometric series to find S9 for the series 12, 6, 3, 3/2 , …
Consider the geometric series,
12, 6, 3, 3/2, …
First term of the series is a1 = 12.
Compute common ratio of the given series as follows:
r = a2/a1
= 6/12
= 1/2
Use the formula for the sum of first ‘n’ terms of a geometric series,
Sn = a1∙(1 – rn)/(1 – r) ...... (1)
Substitute n = 9, r = 1/2 and a1 = 12 in the formula (1) and simplify,
S9 = 12 × {1 – (1/2)9}/(1 – 1/2)
= 12 × {1 – 1/512}/1/2
= (12 × 511/512)/1/2
= 1533/64
Therefore, sum of first 9 terms of the given series is S9 = 1533/64.
Therefore, sum of first 9 terms of the given series is S9 = 1533/64.