How many terms of the AP …
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Posted by Manav Singh Negi 5 years, 11 months ago
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Yogita Ingle 5 years, 11 months ago
a=2
d=2
Sn=210
n=?
Sn=n/2 × [2a+(n-1)d]
210=n/2×[4+(n-1)2]
210×2=n[4+(n-1)2]
420=4n+(n-1)2n
{tex}420=4n+2n^2-2n{/tex}
{tex}420=2(n^2-n){/tex}
{tex}210=n^2-n{/tex}
{tex}n^2-n-210=0{/tex}
{tex}n^2-15n+14n-210=0{/tex}
n(n-15)+14(n-15)=0
(n+14)(n-15)=0
n+14=0. n-15=0
n=-14. n=15
therefore n=15
=>15 terms are needed to get their sum as 210
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