Question 348705
 -x^2+2xy-y^2+16
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= -(x^2-2xy+y^2)+16
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= -(x-y)^2 + 16
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= 16-(x-y)^2
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= [4-(x-y)][4+(x-y)]
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= (4-x+y)(4+x-y)
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Cheers,
Stan H.