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.