Điều kiện tiếp xúc: ![\left\{\begin{matrix} k(x+1)+2=\frac{x-m}{x-1}\\ \\ k=\frac{m-1}{(x-1)^2} \end{matrix}\right.](https://latex.codecogs.com/gif.latex?%5Cleft%5C%7B%5Cbegin%7Bmatrix%7D%20k%28x+1%29+2%3D%5Cfrac%7Bx-m%7D%7Bx-1%7D%5C%5C%20%5C%5C%20k%3D%5Cfrac%7Bm-1%7D%7B%28x-1%29%5E2%7D%20%5Cend%7Bmatrix%7D%5Cright.)
![\Rightarrow \frac{(x+1)(m-1)}{(x-1)^2}+2=(x-m)(x-1)](https://latex.codecogs.com/gif.latex?%5CRightarrow%20%5Cfrac%7B%28x+1%29%28m-1%29%7D%7B%28x-1%29%5E2%7D+2%3D%28x-m%29%28x-1%29)
![\Leftrightarrow x^2+2(m-2)x+1=0\ (*)](https://latex.codecogs.com/gif.latex?%5CLeftrightarrow%20x%5E2+2%28m-2%29x+1%3D0%5C%20%28*%29)
Cách 1:
Để từ A kẻ đúng 1 tiếp tuyến đến đồ thị thì (*) có nghiệm kép khác 1 hoặc có 2 nghiệm phân biệt trong đó có 1 nghiệm bằng 1
![\Leftrightarrow \left\{\begin{matrix} \Delta '=(m-2)^2-1=0\\ -\frac{b}{2a}=2-m\neq 1 \end{matrix}\right. \vee \left\{\begin{matrix} \Delta '=(m-2)^2-1>0\\ 1+2(m-2)+1=0 \end{matrix}\right.](https://latex.codecogs.com/gif.latex?%5CLeftrightarrow%20%5Cleft%5C%7B%5Cbegin%7Bmatrix%7D%20%5CDelta%20%27%3D%28m-2%29%5E2-1%3D0%5C%5C%20-%5Cfrac%7Bb%7D%7B2a%7D%3D2-m%5Cneq%201%20%5Cend%7Bmatrix%7D%5Cright.%20%5Cvee%20%5Cleft%5C%7B%5Cbegin%7Bmatrix%7D%20%5CDelta%20%27%3D%28m-2%29%5E2-1%3E0%5C%5C%201+2%28m-2%29+1%3D0%20%5Cend%7Bmatrix%7D%5Cright.)
![\Leftrightarrow \left\{\begin{matrix} m=3\vee m=1\\ m\neq 1 \end{matrix}\right. \vee \left\{\begin{matrix} m>3 \vee m<1\\ m=1 \end{matrix}\right.](https://latex.codecogs.com/gif.latex?%5CLeftrightarrow%20%5Cleft%5C%7B%5Cbegin%7Bmatrix%7D%20m%3D3%5Cvee%20m%3D1%5C%5C%20m%5Cneq%201%20%5Cend%7Bmatrix%7D%5Cright.%20%5Cvee%20%5Cleft%5C%7B%5Cbegin%7Bmatrix%7D%20m%3E3%20%5Cvee%20m%3C1%5C%5C%20m%3D1%20%5Cend%7Bmatrix%7D%5Cright.)
![\Leftrightarrow x=3](https://latex.codecogs.com/gif.latex?%5CLeftrightarrow%20x%3D3)
Cách 2:
![(*)\Leftrightarrow 2(m-2)=\frac{-x^2-1}{x} (**)](https://latex.codecogs.com/gif.latex?%28*%29%5CLeftrightarrow%202%28m-2%29%3D%5Cfrac%7B-x%5E2-1%7D%7Bx%7D%20%28**%29)
Đặt ![g(x)=\frac{-x^2-1}{x}](https://latex.codecogs.com/gif.latex?g%28x%29%3D%5Cfrac%7B-x%5E2-1%7D%7Bx%7D)
Xét ![g'(x)=-1+\frac{1}{x^2}=0\Leftrightarrow x=\pm1](https://latex.codecogs.com/gif.latex?g%27%28x%29%3D-1+%5Cfrac%7B1%7D%7Bx%5E2%7D%3D0%5CLeftrightarrow%20x%3D%5Cpm1)
Bảng biến thiên:
![](https://i.imgur.com/bGRf3aD.png)
Dựa vào bảng biến thiênĐể
có nghiệm duy nhất khác 1 thì đường thẳng
cắt đồ thị hàm số
tại ![I(-1;2)](https://latex.codecogs.com/gif.latex?I%28-1%3B2%29)
![\Leftrightarrow 2(m-2)=2](https://latex.codecogs.com/gif.latex?%5CLeftrightarrow%202%28m-2%29%3D2)
![\Leftrightarrow m=3](https://latex.codecogs.com/gif.latex?%5CLeftrightarrow%20m%3D3)