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9(x+y)^2-4(x-2y)^2
=[3(x+y)]^2-[2(x-2y)]^2
=[3(x+y)+2(x-2y)][3(x+y)-2(x-2y)]
=(3x+3y+2x-4y)(3x+3y-2x+4y)
=(5x-y)(x+7y)
=[3(x+y)]^2-[2(x-2y)]^2
=[3(x+y)+2(x-2y)][3(x+y)-2(x-2y)]
=(3x+3y+2x-4y)(3x+3y-2x+4y)
=(5x-y)(x+7y)
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