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Let a1, a2, a3, ... be an arithmetic series.  Let Sn denote the sum of the first n terms.  If S1 = 5 and S2 = 15, then find S15.

 Apr 2, 2024
 #1
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S1 = 5

S2 = 15

Then the second term is 10   and  d = 5

15th term  is  5 + 14(5) =  75

 

S15  = [ 15th term + first term ] * no.of terms /  2 =

 [ 75 + 5 ] (15 / 2)   = 80 * 15/2  =  40 * 15  = 600

 

cool cool cool

 Apr 2, 2024

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