Question 40980
find the sum of the first n terms of the given arithmetic series
a1=-13,a7=20.8 (n=7)


S(7) = (7/2)(a(1)+a(7))
S(7) = 3.5(-13+20.8)
S(7) = 3.5*7.8
S(7)= 27.3
Cheers,
stan H.