Question 39509
Find the sum of the first n terms of the given arithmetic series.
a2=-6, a5=-18 (n=8)
A2=A+D=-6......................I
A5=A+4D=-18.................II
3D=-18+6=-12
D=-4
A=-6+4=-2
SN=(N/2){2*-2+(N-1)(-4)}=(N/2){-4-4N+4}= -2N^2
S8=-2*8^2=-128