根号2000乘2001乘2002乘2003加1
=根号（2001-1）乘2001乘（2001+1）乘（2001+2）加1
=根号（2001乘2001-1）乘（2001乘2001+2×2001）加1
=根号（2001^4+2×2001^3-2001^2-2001×2+1
=根号（2001^4+2*2001^3+2001^2-2*2001^2-2*2001+1
=根号[2001^2(2001+1)^2-2*2001(2001+1)+1]
=根号[2001(2001+1)-1]^2
=2001(2001+1)-1
=2001^2+2001-1
=4004001+2001
=4006001