If Sn denotes the sum of n-terms of an A.P. , whose common difference is d and first term is a find:Sn- 2Sn-1 +Sn-2. Hint: An=Sn-Sn-1
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d = d
a = a
an = a+(n-1)d
a+(n-1)d = Sn-Sn-1
a+nd-d = n/2(2a+(n-1)d) - n/2(2a+(n-1)d) -1
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