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x-2y/x+y 减 x-y/x+y 加 -x-4y/x+4y
=[x-2y-(x-y)]/(x+y) 加 -(x+4y)/(x+4y)
=[x-2y-x+y]/(x+y) - (x+4y)/(x+4y)
= -y/(x+y) - 1