The first term of a G.P. is 1. The sum o | Class 11 Mathematics Chapter Sequence and Series, Sequence and Series NCERT Solutions

Question:

The first term of a G.P. is 1. The sum of the third term and fifth term is 90. Find the common ratio of G.P.

Answer:

Let a and r be the first term and the common ratio of the G.P. respectively.

∴ a = 1

a3 = ar2 = r2

a5 = ar4 = r4

∴ r2 + r4 = 90

⇒ r4 + r2 – 90 = 0

rightwards double arrow r squared space equals space fraction numerator minus 1 space plus space square root of 1 plus space 360 end root over denominator 2 end fraction equals fraction numerator minus 1 space plus-or-minus square root of 361 over denominator 2 end fraction equals fraction numerator minus 1 plus-or-minus 19 over denominator 2 end fraction equals minus 10 space o r space 9

∴ r = ± 3                                  ( Taking real roots)

Thus, the common ratio of the G.P. is ±3.


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Comments

  • Subhangi Manisha Mohanty
  • 2019-04-18 07:51:07

Thanks for your answer.i love it thanks once again 😊👌👍💓💖🙏🙋


  • Samyuktha
  • 2018-12-11 19:21:24

Can u write in equation form


  • sree
  • 2018-08-30 22:06:52

Tq😂😂😀


  • Mokshagna
  • 2018-05-20 10:05:18

Can you plese detail answer


  • Mokshagna
  • 2018-05-20 10:03:32

How it will come


Comment(s) on this Question

Welcome to the NCERT Solutions for Class 11 Mathematics - Chapter . This page offers a step-by-step solution to the specific question from Excercise 5 , Question 9: The first term of a G.P. is 1. The sum of the third term and fifth term is 90. Find the common ratio....