以A为åæ åç¹ï¼ABæå¨ç´çº¿ä¸ºx轴建ç«åæ ç³»å¦å¾ï¼
å¨æ¤åæ ç³»éï¼åç¹çåæ å¦ä¸ï¼A(0ï¼0)ï¼B(3ï¼0)ï¼C(9/2ï¼3â3/2)ï¼ D(3/2ï¼3â3/2)ï¼
M(2ï¼0)ï¼åéDM={2-3/2ï¼0-3â3/2}={1/2ï¼-(3/2)â3}ï¼åéAC={9/2ï¼(3/2)â3}ï¼
â´æ°é积DM•AC=(1/2)Ã(9/2)+(-3/2)â3Ã(3/2)â3=(9/4)-(27/4)=-18/4=-9/2;
æ åºéCã
追é®åCDç¹çåæ æ¯æä¹æ±çï¼
追çDç¹çåæ ï¼æ¨ªåæ x=3cos(Ï/3)=3/2ï¼çºµåæ y=3sin(Ï/3)=(3/2)â3ï¼
Cç¹çåæ ï¼æ¨ªåæ x=3+(3/2)=9/2ï¼çºµåæ y=(3/2)â3(ä¸Dç¹ç纵åæ ç¸å)ï¼