Find the points on the curve $y=x^{3}-2 x^{2}-2 x$ at which the tangent lines are parallel to the line $y=2 x-3$.
Let $\left(x_{1}, y_{1}\right)$ be the required point.
Given:
$y=2 x-3$
$\therefore$ Slope of the line $=\frac{d y}{d x}=2$'
$y=x^{3}-2 x^{2}-2 x$
Since $\left(x_{1} y_{1}\right)$ lies on curve, $y_{1}=x_{1}{ }^{3}-2 x_{1}{ }^{2}-2 x_{1} \ldots(1)$
$\Rightarrow\left(\frac{d y}{d x}\right)_{\left(x_{1}, y_{1}\right)}=3 x_{1}^{2}-4 x_{1}-2$
It is given that the tangent and the given line are parallel.
$\therefore$ Slope of the tangent $=$ Slope of the given line
$3 x_{1}^{2}-4 x_{1}-2=2$
$\Rightarrow 3 x_{1}^{2}-4 x_{1}-4=0$
$\Rightarrow 3 x_{1}^{2}-6 x_{1}+2 x_{1}-4=0$
$\Rightarrow 3 x_{1}\left(x_{1}-2\right)+2\left(x_{1}-2\right)=0$
$\Rightarrow\left(x_{1}-2\right)\left(3 x_{1}+2\right)=0$
$\Rightarrow x_{1}=2$ or $x_{1}=\frac{-2}{3}$
Case 1
When $x_{1}=2$
On substituting the value of $x_{1}$ in eq. (1), we get
$y_{1}=8-8-4=-4$
$\therefore\left(x_{1}, y_{1}\right)=(2,-4)$
Case 2
When $x_{1}=\frac{-2}{3}$
On substituting the value of $x_{1}$ in eq. (1), we get
$y_{1}=\frac{-8}{27}-\frac{8}{9}+\frac{4}{3}=\frac{-8-24+36}{27}=\frac{4}{27}$
$\therefore\left(x_{1}, y_{1}\right)=\left(\frac{-2}{3}, \frac{4}{27}\right)$