28: The function f is concave up on [7π/6 + 2kπ,11π/6 + 2kπ] and concave down on [−π/6 + 2kπ,π/2 + 2kπ] and on [π/2 + 2kπ,7π/6 + 2kπ].

Actually, a closer analysis shows that the two intervals of concavity down can be connected, f is concave down on [−π/6 + 2kπ,7π/6 + 2kπ].

Inflection points f (7π/6 + 2kπ) = 7/4 and f (11π/6 + 2kπ) = 7/4.