9:

f is continuous there.
y-intercept: f (0) not possible; x-intercepts: f = 0 not possible.

f (−x) = f (x), hence f is even.
f (x + 2π) = f (x), so f is 2π-periodic.
Limits at endpoints:
Limit at −∞ and ∞ not possible, since D( f ) does not contain any neighborhood of these two points.

Interpretation as asymptotes:
Vertical asymptote at x = 2kπ.

Now determine monotonicity using f ′.

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Answer