问题标题：
分式约分:(y+z-x)/{x^2-(y+z)^2}
问题描述：
分式约分:(y+z-x)/{x^2-(y+z)^2}
姜立军回答：
(y+z-x)/{x^2-(y+z)^2}
=(y+z-x)/[(x-y-z)(x+y+z)]
=-(x-y-z)/[(x-y-z)(x+y+z)]
=-1/(x+y+z)
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