Question:
Determine the equation (s) of tangent (s) line to the curve $y=4 x^{3}-3 x+5$ which are perpendicular to the line $9 y+x+3=0$.
Solution:
finding the slope of the tangent by differentiating the curve
$\frac{d y}{d x}=12 x^{2}-3$
$\mathrm{m}($ tangent $)=12 \mathrm{x}^{2}-3$
the slope of given line is $-\frac{1}{9}$, so the slope of line perpendicular to it is 9
$12 x^{2}-3=9$
$x=1$ or $-1$
since this point lies on the curve, we can find y by substituting $x$
$y=6$ or 4
therefore, the equation of the tangent is given by equation of tangent is given by
$y-y_{1}=m(\operatorname{tangent})\left(x-x_{1}\right)$
$y-6=9(x-1)$
or
$y-4=9(x+1)$