f(x)=2x³-3(a+1)x²+6ax+8
f'(x)=6x²-6(a+1)x+6a
xâ(-â,0)为å¢å½æ°âf'(x)â¥0 ææç«
令g(x)=f'(x)=6x²-6(a+1)x+6a=6[x-(a+1)/2]²-1.5a²+3a-1
â :åºé´xâ(-â,0)å å«å¯¹ç§°è½´x=(a+1)/2ï¼å³(a+1)/2â¤0âaâ¤-1æ¶
顶ç¹-1.5a²+3a-1>0âæ 解
â¡åºé´xâ(-â,0)å¨å¯¹ç§°è½´x=(a+1)/2ç左侧å³(a+1)/2>0âa>-1æ¶ï¼å½æ°g(x)åè°éå¢,g(x)>g(0)=6aâ¥0âaâ¥0
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