evaluate 13 + 33 + 53 + 73 + ...... upto n terms.
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a=13
d=20
nth term=13+(n-1)20
=-7+20n
Sn=n/2(a+l)
=n/2(13+(-7+20n))
=n/2(6+20n)
=6n+20n^2/2
=3n+10n^2
d=20
nth term=13+(n-1)20
=-7+20n
Sn=n/2(a+l)
=n/2(13+(-7+20n))
=n/2(6+20n)
=6n+20n^2/2
=3n+10n^2
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