设y-1=k(x-2) k不等于0
y-kx+2k-1=0
Q(1,-2)
|-2-k+2k-1|/根号下(1+k*k)=根号2
k*k-6k+9=2+2k*k
k*k+6k-7=0
k=1或-7
直线方程为 y=x-1或者y=-7x+15