>>39
a_{2n+2} - 4/5 = -1/4 ( a_{2n} - 4/5)
= (-1/4)^2 (a_{2n-2} - 4/5)
= (-1/4)^3 (a_{2n-4} - 4/5)
...
= (-1/4)^(n+1) (a_0 -4/5)

これで分かる？