SOLUTION: Find s15 for the series -1+-3+-5+-7+... Thanks!

Question 1069752: Find s15 for the series -1+-3+-5+-7+...
Thanks!
Answer by ikleyn(52790) (Show Source): You can put this solution on YOUR website!
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Find s15 for the series -1+-3+-5+-7+...
Thanks!
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-1 + -3 + -5 + -7 + . . . = -(1 + 3 + 5 + . . . ) = 


   This is the sum of arithmetic progression.

   Use the formula  1 + 3 + 5 + . . . + (2n-1) = .
   In your case n = 15.

   See the lessons

           - Arithmetic progressions

           - The proofs of the formulas for arithmetic progressions 

           - Problems on arithmetic progressions  

   in this site


=  = -225.

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