Question

Find S15 for the series -1 + -3 + -5 + -7 +...

Answer 1

Hello :
the sum is  arithmetic secquence of common -diff is :
(-7)-(-5) =(-5)-(-3) = (-3)-(-1) =-2 = d
  the first term is : a1 = -1
the formula of the sum :  S= (n/2)(a1 +an)
n is number of term       an = a1+(n-1)d