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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.

 Mar 20, 2024
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First term  = 1  =  a

Second term  = 14

15th term =  1 + 14 * 13  = 183

 

S15 =    [ first term + last term ]  (no.of terms / 2)  =

 

]1 + 183 ] / (15/2)  =

 

1380  = S15

 

cool cool cool

 Mar 20, 2024
edited by CPhill  Mar 20, 2024

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