1,a>b,a>c,a+a>b+c,3a>a+b+c=0,a>0
a>b,a+a+c>a+c+b=0,c/a>-2
b>c,a+c+b>a+c+c,0>a+2c,c/a<-1/2
c/a的范围(-2,-1/2)
2,y^2=(x+3z)^2/4
y^2/xz=(x/z+6+9z/x)/4>=3
y方/xz的最小值是3