Answer:
A₍ₙ₎ = - 74 + (n-1) x 6
Step-by-step explanation:
A₍ₙ₎ = A₍₁₎ + (n-1) x d
A₍₁₈₎ = 28 = A₍₁₎ + 17 x d ..... (1)
A₍₄₀₎ = 160 = A₍₁₎ + 39 x d .....(2)
(2)-(1) 132 = 22d
d = 6
A₍₁₎ = 28 - 17 x 6 = -74
formula: A₍ₙ₎ = - 74 + (n-1) x 6
