题目内容
先化简，再求值：[（3m+n）（3m-n）+n（n-6m）]÷3m，其中m=
，n=
．
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[（3m+n）（3m-n）+n（n-6m）]÷3m，
=[9m2-n2+n2-6mn]÷3m
=[9m2-6mn]÷3m
=3m-2n，
当m=
，n=
时，原式=3×
-2×
=
．
=[9m2-n2+n2-6mn]÷3m
=[9m2-6mn]÷3m
=3m-2n，
当m=
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