Find the point on the curve
Question:

Find the point on the curve $y=3 x^{2}+4$ at which the tangent is perpendicular to the line whose slop is $-\frac{1}{6}$.

Solution:

Let $\left(x_{1}, y_{1}\right)$ be the required point.

Slope of the given line $=\frac{-1}{6}$

$\therefore$ Slope of the line perpendicular to it $=6$

Since, the point lies on the curve.

Hence, $y_{1}=3 x_{1}^{2}+4$

Now, $y=3 x^{2}+4$

$\therefore \frac{d y}{d x}=6 x$

Now,

Slope of the tangent at $\left(x_{1}, y_{1}\right)=\left(\frac{d y}{d x}\right)_{\left(x_{1}, y_{1}\right)}=6 x_{1}$

Slope of the tangent at $\left(x_{1}, y_{1}\right)=$ Slope of the given line [Given]

$\therefore 6 x_{1}=6$

$\Rightarrow x_{1}=1$

and

$y_{1}=3 x_{1}^{2}+4=3+4=7$

Thus, the required point is $(1,7)$

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