Question:
At what points on the curve $y=x^{2}-4 x+5$ is the tangent perpendicular to the line $2 y+x=7 ?$
Solution:
Let $\left(x_{1}, y_{1}\right)$ be the required point.
Slope of the given line $=\frac{-1}{2}$
Slope of the line perpendicular to this line $=2$
Since, the point lies on the curve.
Hence, $y_{1}=x_{1}{ }^{2}-4 x_{1}+5 \quad \ldots(1)$
Now, $y=x^{2}-4 x+5$
$\therefore \frac{d y}{d x}=2 x-4$
Now,
Slope of the tangent at $\left(x_{1}, y_{1}\right)=\left(\frac{d y}{d x}\right)_{\left(x_{1}, y_{1}\right)}=2 x_{1}-4$
Slope of the tangent at $\left(x_{1}, y_{1}\right)=$ Slope of the given line [Given]
$\therefore 2 x_{1}-4=2$
$\Rightarrow 2 x_{1}=6$
$\Rightarrow x_{1}=3$
Also,
$y_{1}=9-12+5=2$ [From eq. (1)]
Thus, the required point is $(3,2)$.