Check sibling questions

Ex 9.3, 8 - Find sum to n terms root 7, root 21 - Chapter 9 - Geometric Progression(GP): Formulae based

Ex 9.3, 8 - Chapter 9 Class 11 Sequences and Series - Part 2
Ex 9.3, 8 - Chapter 9 Class 11 Sequences and Series - Part 3

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Ex 9.3, 8 Find the sum to n terms in the geometric progression 7 , 21 ,3 7 7 , 21 ,3 7 Here, First term a = 7 Common ratio r = 21/ 7 = (7 3)/ 7 = ( 7 3 )/ 7 = 3 So r = 3 1.73 Since, r > 1 Sn = ( ( ^ 1))/( 1) Sn = ( ( ^ 1))/( 1) where Sn = sum of n terms of GP n is the number of terms a is the first term r is the common ratio Now, Sum of n terms = ( ( ^ 1))/( 1) Putting values a = 7 , r = 3 Sn = ( 7 (( 3)^ 1))/( 3 1) Rationalizing the same = ( 7 (( 3)n 1 )])/( 3 1) x ( 3 + 1)/( 3 + 1) = ( 7 ( 3 1) ( 3+ 1))/(( 3 1) ( 3+ 1)) = ( 7 ( 3 1) ( 3+ 1))/(( 3 1) ( 3+ 1)) Using a2 b2 = (a + b)(a b) = ( 7 (3^(1/2 ) 1)( 3 +1))/(( 3)2 1^2 ) =( 7 (3^( /2) 1) ( 3 + 1))/2 = 7/2( 3+1) (3^( /2) 1) Hence sum of n term is 7/2( 3+1) (3^( /2) 1)

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Davneet Singh

Davneet Singh has done his B.Tech from Indian Institute of Technology, Kanpur. He has been teaching from the past 12 years. He provides courses for Maths, Science, Social Science, Physics, Chemistry, Computer Science at Teachoo.