设A(3cosa,3sina),B(3cosb,3sinb),则中点坐标：(x,y)满足：x=3(cosa+cosb)/2,y=3(sina+sinb)/2x^2+y^2=9/4[(cosa+cosb)^2+(sina+sinb)^2]=9/4*(2+2(cosacosb+sinasinb))9(cosacosb+sinasinb)=2(x^2+y^2)-9-1=(3sina-...