Question:
If in an infinite G.P., first term is equal to 10 times the sum of all successive terms, then its common ratio is
(a) 1/10
(b) 1/11
(c) 1/9.
(d) 1/20
Solution:
(b) $\frac{1}{11}$
Let the first term of the G.P. be a.
Let its common ratio be r.
According to the question, we have:
First term = 10 [Sum of all successive terms]
$a=10\left(\frac{a r}{1-r}\right)$
$\Rightarrow a-a r=10 a r$
$\Rightarrow 11 a r=a$
$\Rightarrow r=\frac{a}{11 a}=\frac{1}{11}$