摘要：17．设x＜y＜0.试比较(x2+y2)(x-y)与(x2-y2)·(x+y)的大小． 解:(x2+y2)(x-y)-(x2-y2)(x+y) ＝(x-y) ＝-2xy(x-y). ∵x＜y＜0.∴xy＞0.x-y＜0.∴-2xy(x-y)＞0. ∴(x2+y2)(x-y)＞(x2-y2)(x+y)．
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