a, A₁, A₂, A₃, ..., Aₙ, b If it’s an arithmetic progression, then:

Aₙ = a + n(b − a)/n

Aₙ = a + n(b − a)/(n + 1)

Aₙ = a − n(b + a)/(n + 1)

Aₙ = b + n(a − b)/n

Aₙ = a + n × d, where d = common difference

Here, A₁ = a + d, A₂ = a + 2d, ..., b = a + (n+1)d

So,

Aₙ = a + n × [(b − a)/(n + 1)]