Question 1069752
.
Find s15 for the series -1+-3+-5+-7+...

Thanks!
~~~~~~~~~~~~~~~~~~~~~~~~~~`


<pre>
-1 + -3 + -5 + -7 + . . . = -(1 + 3 + 5 + . . . ) = 


   This is the sum of <U>arithmetic progression</U>.

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

   See the lessons

       &nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=http://www.algebra.com/algebra/homework/Sequences-and-series/Arithmetic-progressions.lesson>Arithmetic progressions</A>

       &nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=http://www.algebra.com/algebra/homework/Sequences-and-series/The-proofs-of-the-formulas-for-arithmetic-progressions.lesson>The proofs of the formulas for arithmetic progressions</A> 

       &nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=http://www.algebra.com/algebra/homework/Sequences-and-series/Problems-on-arithmetic-progressions.lesson>Problems on arithmetic progressions</A>  

   in this site


= {{{-(15^2)}}} = -225.
</pre>