18: The second derivative is

f ′′ = cos(x).

Dividing points: f ′′ = 0 gives x = π/2 + kπ; there are no points in D( f ) where f ′′ does not exist.

Inflection points: f (π/2 + kπ) = π/2 + kπ.

Now draw coordinate axes and mark all points and trends we learned so far.

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