令1/a=2/x=3/y=4/zx=2ay=3az=4a4x^+2yz+z^/x+y-z × x-z-y/8x^+4yz+2z^=4x^+2yz+z^/x+y-z × x-z-y/2(4x^+2yz+z^)=(x-z-y)/[2(x+y-z)]=(2a-4a-3a)/[2(2a+3a-4a)]=-5/2