过A直线 y-1=k(x+1)
y=kx+(1+k)
(x-1)^2+[kx+(3+k)]^2=4
(k^2+1)x^2+(6k+2k^2-2)x+k^2+6k+6=0
判别式
(6k+2k^2-2)^2-4(k^2+1)(k^2+6k+6)=0
(4k^4+36k^2+4+24k^3-8k^2-24k)-(4k^4+24k^3+25k^2+24k+24)=0
28k^2-24k+4-25k^2-24k-24=0
3k^2-48k-20=0
3(k-8)^2=20+192
k=8±√(212/3)
y=[8+√(212/3)]x+9+√(212/3) 或 y=[8-√(212/3)]x+9-√(212/3)