C:(x-3)^2+y^2=4,r=2
C(3,0),M(x,y)
x=3+2cosα
y=2sinα
k(AB)=y/x=2sinα/(3+2cosα)
k(CM)=y/(x-3)=sinα/cosα
k(AB)*k(CM)=-1
x=1.5+1.5*cosα
y=1.5*sinα