设a=1994
则：（1994^2-2000)(1994^2+3985)*1995/(1991*1993*1996*1997)
=(a^2-a-6)(a^2+2a-3)*(a+1)/[(a-3)*(a-1)(a+2)(a+3)]
=(a-3)(a+2)(a+3)(a-1)(a+1)/[(a-3)*(a-1)(a+2)(a+3)]
=a+1
=1995