令a=1996
则1996*1997*1998*1999+1
=a(a+1)(a+2)(a+3)+1
=[a(a+3)][(a+1)(a+2)+1
=(a²+3a)[(a²+3a)+2]+1
=(a²+3a)²+2(a²+3a)+1
=(a²+3a+1)²
所以原式=a²+3a+1=1996²+3×1996+1=3990005