JEE Mains · Maths · STD 11 - 8. sequence and series
If the sum of the second, fourth and sixth terms of a G.P. of positive terms is 21 and the sum of its eighth, tenth and twelfth terms is 15309 , then the sum of its first nine terms is :
- A 760
- B 755
- C 750
- D 757
Answer & Solution
Correct Answer
(D) 757
Step-by-step Solution
Detailed explanation
\begin{aligned} & \mathrm{ar}+\mathrm{ar}^3+\mathrm{ar}^5=21, \quad \mathrm{ar}^7+\mathrm{ar}^9+\mathrm{ar}^{11}=15309 \\ & \Rightarrow \operatorname{ar}\left(1+\mathrm{r}^2+\mathrm{r}^4\right)=21, \quad…
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