a2=1/(1-a1)=1/[1-(-1/2)]=2/3
a3=1/(1-a2)=1/(1-2/3)=3
a4=1/(1-a3)=1/(1-3)=-1/2=a1
规律：数列从第1项开始,按-1/2,2/3,3循环,每3项循环一次.
a10·a11·a12/a1
=a(3·3+1)·a(3·3+2)·a(3·3+3)/a1
=a1·a2·a3/a1
=a2·a3
=(2/3)·3
=2