9(x-y)²-y²,=[3(x-y)]²-y²=[3(x-y)+y][3(x-y)-y]=(3x-2y)(3x-4y)9a²-4(b＋c)²,=(3a)²-[2(b+c)]²=[3a+2(b+c)][3a-2(b+c)]=(3a+2b+2c)(3a-2b-2c)（x＋y＋z)²-(x-y-z)
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