find the sum of the series 3+18+57+132+255 - - - upto n terms.
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3
a=3
d=18-3=15
sum= n(2a+(n-1)×d)/2
so,
sum=n(6+(n-1)×15/2
d=18-3=15
sum= n(2a+(n-1)×d)/2
so,
sum=n(6+(n-1)×15/2
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