# What is the equation of the line normal to  f(x)=xtanx at  x=pi/2?

$\frac{\pi}{2}$ is not in the domain of $\tan x$, so it is not in the domain of $f$.
There is no point on the graph with $x = \frac{\pi}{2}$, so there is no line normal to the graph at $x = \frac{\pi}{2}$.