(x-y+9)(x+y-9)则么算

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(x-y+9)(x+y-9)则么算
(x-y+9)(x+y-9)则么算

(x-y+9)(x+y-9)则么算
(x-y+9)(x+y-9)
=x²-(y-9)²
=x²-(y²-18y+81)
=x²-y²+18y-81

=[x-(y-9)][x+(y-9)]

(x-y+9)(x+y-9)
=(x-(y-9))(x+(y-9))
=x^2-(y-9)^2
==x^2-y^2+18y-81

把y-9看成一个整体,然后用平方差公式算

原式=[x-(y-9)][x+(y-9)]
=x^2-(y-9)^2

(x-y+9)(x+y-9)
=(x-(y-9))(x+(y-9))
=x^2-(y-9)^2
==x^2-y^2+18y-81