y=a(x+3)(x-1)=ax^2+2ax-3a
对称轴x=-1
C(0,-3a) D(-1,-4a)
ç´çº¿CD: y=ax-3a
æ以Bå°ç´çº¿CDè·ç¦»=2a/æ ¹å·ï¼a^2+1ï¼
ç±äºGæ¯ACBçéå¿ï¼æ以Gç纵åæ æ¯1/3(0+0+-3a)=-a
æ以ä¸è§å½¢AGBçé¢ç§¯S=1/2 *a*4=2a
H/S=1/æ ¹å·(a^2+1)
è¿éï¼è¦æ±è§ACBå°äºçäº90°ãæ以AC^2+BC^2>=AB^2
å³[3^2+(-3a)^2]+[1^2+(-3a)^2]>=4^2
解ä¸é¢è¿ä¸ªä¸çå¼
å¾å°açèå´ï¼a^2>=1/3
æ以H/S<=1/æ ¹å·ï¼4/3ï¼=(æ ¹å·3)/2
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