How many terms of the a.p 1,4,7 are needed to make the sum 51
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Answered by
54
first term (a)=1
common difference (d)=3
we know
Sn=n/2 {2a+(n-1) d}
=>51=n/2 {2.1+(n-1) 3}=n/2 {3n-1}
=>3n^2-n=102
=> 3n^2-n-102=0
use quadratic formula
n=6,-17/3
but n =-17/3 is not possible
so, n=6
common difference (d)=3
we know
Sn=n/2 {2a+(n-1) d}
=>51=n/2 {2.1+(n-1) 3}=n/2 {3n-1}
=>3n^2-n=102
=> 3n^2-n-102=0
use quadratic formula
n=6,-17/3
but n =-17/3 is not possible
so, n=6
abhi178:
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