For the given equation ⇒⇒⇒

and the given range 0 ≤ x ≤ π
We should note that the zeros of the denominator will be at
cos x = 0
∴

So, the given equation is undefined at π/2
So,
the students error is the division by zero.
See the attached figure which represents the graph of the given equation and we can see it is undefined at π/2