1.已知x+y= -4,xy= -12,则y+1/x+1 + x+1/y+1 等于?
2.若 1/a + 1/b = 1/a-b,则b/a - a/b 等于?
3.若x / y+z = a,y / x+z = b ,z / x+y =c,且x+y+z ≠ 0,则 a/1+a + b/1+b + c/1+C等于?
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