Sequence And Series Question 607
Question: If the sum of an infinite G.P. be 9 and the sum of first two terms be 5, then the common ratio is
Options:
1/3
3/2
3/4
2/3
Show Answer
Answer:
Correct Answer: D
Solution:
Infinite series is $ a,\ ar,\ ar^{2}……..\infty $ Sum $ S=\frac{a}{1-r}=9\Rightarrow a=9(1-r) $ ?..(i) and $ a+ar=5 $
$ \Rightarrow a(1+r)=5\Rightarrow a(1-r)(1+r)=5 $ [By equation (i)] $ \frac{2^{n}-1}{{2^{n-1}}}=\frac{255}{128} $ .
BETA
