a(1) = 1/1
a(2) = 1/2
a(3) = 1/(1+1/2) = 2/3
a(4) = 1/(1+2/3) = 3/5
a(5) = 1/(1+3/5) = 5/8
・・・・・
分母は
b(n) = b(n-1) + b(n-2)
b(1) = 1 , b(2) = 2