In the A.P.7,14,21.....How many terms are to be considered for getting sum 5740?
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Answered by
73
given series ,
7 , 14 , 21 , .......
first term =7
common difference =7
use formula ,
Sn=n/2 {2a +(n-1) d}
5740 =n/2 {2.7 +(n-1) 7}
11480 =14n + 7n^2 - 7n
=7n + 7n^2
n^2 + n =1640
n^2 + n -1640= 0
use quadriatic formula ,
n={ -1 +_root (1+6560)}/2
=(-1+_81)/2 =40, -41
hence n=40
7 , 14 , 21 , .......
first term =7
common difference =7
use formula ,
Sn=n/2 {2a +(n-1) d}
5740 =n/2 {2.7 +(n-1) 7}
11480 =14n + 7n^2 - 7n
=7n + 7n^2
n^2 + n =1640
n^2 + n -1640= 0
use quadriatic formula ,
n={ -1 +_root (1+6560)}/2
=(-1+_81)/2 =40, -41
hence n=40
Answered by
46
Last me factorization kiya quadratic eq^n bana ke
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