n≥2时,nan=(n+1)·a(n-1)
an/(n+1)=a(n-1) /n
a1/2=1/2,数列{an/(n+1)}是各项均为1/2的常数数列
an/(n+1)=1/2
an=(n+1)/2
令an=9
(n+1)/2=9
n+1=18
n=17
n≥2时,nan=(n+1)·a(n-1)
an/(n+1)=a(n-1) /n
a1/2=1/2,数列{an/(n+1)}是各项均为1/2的常数数列
an/(n+1)=1/2
an=(n+1)/2
令an=9
(n+1)/2=9
n+1=18
n=17