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

-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.