Find the sum of the first 15 terms of the arithmetic sequence

Question

a_{n = 14 - 6n}

Answer 1

a_{1 =} 8, a₂ = 2, a₃= -4....

a = first term = 8

d = common difference = -6

number of terms = n = 15

S₁₅ = n/2 (2a +(n-1)d)

Substitute value of a, d, n in S₁₅ formula

Answer 2

a_{n = 14 - 6n, then a1 = 8, a15 = -76}

_{Sn = (n/2)(a1 + an) = (15/2) ( 8 - 76) = -510}