find the sum of the series upto n terms : 1-3-7-11.....
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d=-4
a=1
S n = n/2{2a+(n-1)d}
= n/2{2-4n+4}
= n/2(6-4n)
= 3n-2n² ----- Answer.
a=1
S n = n/2{2a+(n-1)d}
= n/2{2-4n+4}
= n/2(6-4n)
= 3n-2n² ----- Answer.
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