圆心C在过切点A与直线x+y-√2=0垂直的直线L上x+y-√2=0斜率K=-1，L斜率KLKL*K=-1，KL=1，L：y=xC为y=x和y=-2x交点C(0,0)半径R^2=（√2）^2+(√2)^2=4C方程x^2+y^2=42）AB：Y=2X-√2，AD=2X+3√2带入圆x^2+y^2=4B(-√2/5,-7√2/5),D(7√2/5,√2/5)KBD=1
