方法一、直接列方程求解(最基本但却是很笨的办法)设过A(4,0)的直线方程:y=kx+b则有,4k+b=0,b=-4ky=kx-4kx^2+K^2(x-4)^2=4x^2(1+k^2)-8k^2x+16k^2-4=0x1+x2=8k^2/(1+k^2)相切的条件:k=±√3/3x^2+(1/3)(x-4)^2=4,即,4x^2-8x+4=0,x=1BC中点的轨迹: 令x=(x1+x2)/2,2x+2xk^2=8k^2,k^2=x/(4-x),k=±√[x(4-x)]/(x-4)y=k(x-4)=±√[x(4-x)]y^2=4x-x^2(x-2)^2+y^2-4=0即,(x-2)^2+y^2
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