We have the formula,
s = ut + (at²) / 2
Here the particle starts sliding down, so it was in rest, hence the initial velocity, u = 0.
Here a = mg sin θ, where m is the mass of the particle, g acceleration due to gravity and θ the angle of inclination of the inclined plane. But we take it as 'a' only.
So, for 'n - 1' seconds,
s = a(n - 1)² / 2
And for 'n' seconds,
s = an² / 2
Given that S_n = s(n) - s(n - 1), i.e.,
S_n = [n² - (n - 1)²] a / 2
S_n = (2n - 1) a / 2
So we can have S_(n - 1) = (2n - 3) a / 2
Hence the ratio will be,
S_n / S_(n - 1) = (2n - 1) / (2n - 3)