Question

1. One of the formulas for finding the partial sum of an arithmetic sequence is Sn = n(a1 + an)
True

2. The value of a probability must be in the interval [0, 1]?
True

Answer 1

#1. Nope. It's n/2 (a1+an)
Always check your answer. It's easy here.
if a=d=1,
S2 = 1+2 = 2/2 (1+2) not 2(1+2)

#2. ok