x=√5+2
所以（x-2）²=5
即x²-4x-1=0,即x²=4x+1
x³+3x²-5x+1999
=x（4x+1）+3（4x+1）-5x+1999
=4x²+8x+2002
=4（4x+1）+8x+2002
=24x+2006
=24*（√5+2）+2006
=2054+24√5