Question

Find the indicated sum for the arithmetic series.
S16 for 12 + 7 + 2 + (-3) + ...

Answer 1

a1=12
d=common difference=-5
a16=a1+(16-1)*d
a16=12+15*-5
a16=-63
sum of n terms=n(a1+an)/2
n=16
a1=12
a16=-63
sum=16(12-63)/2
sum=8*(-51)=-408

Answer 2

a1=first term
a16=sixteenth term of the series