Question:
At what points on the curve $y=2 x^{2}-x+1$ is the tangent parallel to the line $y=3 x+4 ?$
Solution:
Let $\left(x_{1}, y_{1}\right)$ be the required point.
The slope of line $y=3 x+4$ is 3 .
Since, the point lies on the curve.
Hence, $y_{1}=2 x_{1}^{2}-x_{1}+1$
Now, $y=2 x^{2}-x+1$
$\frac{d y}{d x}=4 x-1$
Now,
Slope of the tangent at $\left(x_{1}, y_{1}\right)=\left(\frac{d y}{d x}\right)_{\left(x_{1}, y_{1}\right)}=4 x_{1}-1$
Slope of the tangent at $\left(x_{1}, y_{1}\right)=$ Slope of the given line [Given]
$\therefore 4 x_{1}-1=3$
$\Rightarrow 4 x_{1}=4$
$\Rightarrow x_{1}=1$
and
$y_{1}=2 x_{1}^{2}-x_{1}+1=2-1+1=2$
Thus, the required point is $(1,2)$.