>>649

(a[n+1] - 1) / (a[n+1] + 2) = 2/5 * (a[n] - 1) / (a[n] + 2) となる。

r = (a[n+1]・a[n] + 2a[n]+1 - a[n] - 2) / (a[n+1]・a[n] - a[n]+1 + 2a[n] - 2)