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Learn in your speed, with individual attention - Teachoo Maths 1-on-1 Class


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Ex 8.2, 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 14 years. He provides courses for Maths, Science, Social Science, Physics, Chemistry, Computer Science at Teachoo.