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问题: 求积

数列{an}中,a<n+1>=(1+an)/(1-an),a1=2,
求{an}前2007项的积.

解答:

a1=2
a2=(1+2)/(1-2)=-3
a3=(1-3)/(1+3)=-1/2
a4=(1/2)/(3/2)=1/3
a5=(4/3)/(2/3)=2=a1
{an}是周期函数,周期T=4
a1a2a3a4=2(-3)(-1/2)(1/3)=1
{an}前2007项的积={an}前3项的积
=2(-3)(-1/2)
=3