m三次方-4m=m(m²-4）=m(m+2)(m-2)
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m三次方n-9mn=mn(m²-9)=mn(m+3)(m-3)
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3m(2x-y)平方-3mn二次方=3m[(2x-y)²-n²]=3m(2x-y+n)(2x-y-n)