1/x+1/y=1/6,1/y+1/z=1/9,1/z+1/x=1/15
2(1/x+1/y+1/z)=1/6+1/9+1/15=31/90
1/x+1/y+1/z=31/180
(xy+yz+zx)/xyz=31/180
xyz/（xy+yz+zx）=180/31