根号不小于0
所以y>=0
y²=1-x+2√(1-x)(x+3)+x+3
=4+2√(-x²-2x+3)
-x²-2x+3=-(x+1)²+4
定义域 1-x>=0,x+3>=0
-3<=x<=1
所以x=-1,-x²-2x+3最大=4
x=-3,x=1,-x²-2x+3最小=0
所以0<=-x²-2x+3<=4
0<=√(-x²-2x+3)<=2
4<=4+2√(-x²-2x+3)<=4+2×2=8
4<=y²<=8
y>=0
2<=y<=2√2
M=2√2.N=2
M/N=√2