15: The function f is increasing on [−π/2 + 2kπ,π/6 + 2kπ] and on [π/2 + 2kπ,5π/6 + 2kπ] and decreasing on [π/6 + 2kπ,π/2 + 2kπ] and on [5π/6 + 2kπ,3π/2 + 2kπ].
Local maxima f (π/6 + 2kπ) = 1/4 and f (5π/6 + 2kπ) = 1/4.
Local minima f (π/2 + 2kπ) = 0 and f (3π/2 + 2kπ) = −2.