Browse by Stream
QnA
Home
QnA
Go To Your Study Dashboard

Get Answers to all your Questions

header-bg qa
  • Home
  • Engineering
  • Let the sum of an infinite G.P., whose first term is a and the common ratio is r, be 5 . Let the sum of its first five terms be <img alt="\frac{98}{25}" src="/latex-image/?%5Cfrac%7B98%7D%7B25%7D"

Let the sum of an infinite G.P., whose first term is a and the common ratio is r, be 5 . Let the sum of its first five terms be \frac{98}{25}. Then the sum of the first 21 terms of an AP, whose first term is \mathrm{10 a r, n^{\text {th }}} term is \mathrm{a_{n}} and the common difference is \mathrm{10 a^{2}}, is equal to :
 

Option: 1

21 \mathrm{a}_{11}


Option: 2

22 \mathrm{a}_{11}


Option: 3

15 \mathrm{a}_{16}


Option: 4

14 \mathrm{a}_{16}


Answers (1)

best_answer

 

\begin{aligned} &\text { Sum of 21 terms } \\ &\mathrm{=\frac{21}{2}\left[2(10 a r)+20 \cdot \text { loar }^{2}\right] }\\ & \mathrm{=21 r\left(10 a r+1.00 a^{2}\right) }\\ & \mathrm{=21\left[a_{1}+(11-1) \cdot 10 a^{2}\right)} \\ & \mathrm{=21\left(a_{1}+(11-1) d\right) }\\ &\mathrm{=21 \cdot a_{11}} \\ &\therefore \text { option (A) } \end{aligned}

Posted by

Rakesh

View full answer

JEE Main high-scoring chapters and topics

Study 40% syllabus and score up to 100% marks in JEE

Similar Questions

Trending Articles/News

anam-khan

BTech aspirant from north-east or UT? Apply for CSAB NEUT counselling 2025 today
anam-khan

JoSAA round 2 cut-off 2025 out; BTech closing ranks for top engineering courses at IITs
anam-khan

Goa Engineering Admission 2025: 1,415 eligible for BE seats; merit list on June 27

Latest Question