>>445

f(0) = 5,
f(1) = 5 f(0) + f(0) = 6 f(0),
f(2) = 5 f(1) + f(1) = 6 f(1), ...
f(n+1) = 5 f(n) + f(n) = 6 f(n)
となる. f(n+1) / f(n) = 6 だから
f(n) = 5 * 6^n