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最佳答案:> diff(y*f(y), y);f(y)+y*(diff(f(y), y))> diff(%, y);2*(diff(f(y), y))+y*(diff(f
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最佳答案:y'=3x^2-6x-9=3(x^2-2x-3)=3(x-3)(x+1)=0,得极值点x=3,-1y"=6x-6x=3时,y"(3)=12>0,因此x=3为极小