In order to find the extrema, we need to solve the equation
$$\frac{d}{d x} f{\left(x \right)} = 0$$
(the derivative equals zero),
and the roots of this equation are the extrema of this function:
$$\frac{d}{d x} f{\left(x \right)} = $$
the first derivative$$\frac{x \sin{\left(x \right)}}{\cos^{2}{\left(x \right)}} + \frac{1}{\cos{\left(x \right)}} = 0$$
Solve this equationThe roots of this equation
$$x_{1} = -84.811211299318$$
$$x_{2} = -25.0929104121121$$
$$x_{3} = 72.2427897046973$$
$$x_{4} = -18.7964043662102$$
$$x_{5} = -47.1026627703624$$
$$x_{6} = -6.12125046689807$$
$$x_{7} = -59.6735041304405$$
$$x_{8} = 43.9595528888955$$
$$x_{9} = -78.5270825679419$$
$$x_{10} = -12.4864543952238$$
$$x_{11} = -9.31786646179107$$
$$x_{12} = -56.5309801938186$$
$$x_{13} = 100.521017074687$$
$$x_{14} = -53.3883466217256$$
$$x_{15} = 21.945612879981$$
$$x_{16} = -15.644128370333$$
$$x_{17} = -21.945612879981$$
$$x_{18} = 9.31786646179107$$
$$x_{19} = -34.5285657554621$$
$$x_{20} = -100.521017074687$$
$$x_{21} = 50.2455828375744$$
$$x_{22} = 78.5270825679419$$
$$x_{23} = 53.3883466217256$$
$$x_{24} = 56.5309801938186$$
$$x_{25} = 25.0929104121121$$
$$x_{26} = 18.7964043662102$$
$$x_{27} = 15.644128370333$$
$$x_{28} = 2.79838604578389$$
$$x_{29} = 47.1026627703624$$
$$x_{30} = 91.0952098694071$$
$$x_{31} = -72.2427897046973$$
$$x_{32} = -28.2389365752603$$
$$x_{33} = 94.2371684817036$$
$$x_{34} = 69.100567727981$$
$$x_{35} = 34.5285657554621$$
$$x_{36} = -87.9532251106725$$
$$x_{37} = -97.3791034786112$$
$$x_{38} = 31.3840740178899$$
$$x_{39} = -62.8159348889734$$
$$x_{40} = -94.2371684817036$$
$$x_{41} = 62.8159348889734$$
$$x_{42} = -81.6691650818489$$
$$x_{43} = 97.3791034786112$$
$$x_{44} = 28.2389365752603$$
$$x_{45} = 75.3849592185347$$
$$x_{46} = 6.12125046689807$$
$$x_{47} = -37.672573565113$$
$$x_{48} = 81.6691650818489$$
$$x_{49} = -31.3840740178899$$
$$x_{50} = -40.8162093266346$$
$$x_{51} = -43.9595528888955$$
$$x_{52} = -69.100567727981$$
$$x_{53} = -65.9582857893902$$
$$x_{54} = 40.8162093266346$$
$$x_{55} = -91.0952098694071$$
$$x_{56} = -75.3849592185347$$
$$x_{57} = 87.9532251106725$$
$$x_{58} = -2.79838604578389$$
$$x_{59} = 12.4864543952238$$
$$x_{60} = 59.6735041304405$$
$$x_{61} = 37.672573565113$$
$$x_{62} = 84.811211299318$$
$$x_{63} = 65.9582857893902$$
$$x_{64} = -50.2455828375744$$
The values of the extrema at the points:
(-84.811211299318, 84.817106541414)
(-25.0929104121121, -25.1128284538059)
(72.2427897046973, -72.2497104791231)
(-18.7964043662102, -18.822986402218)
(-47.1026627703624, 47.1132766856486)
(-6.12125046689807, -6.20239528557313)
(-59.6735041304405, 59.6818824703587)
(43.9595528888955, 43.9709255098366)
(-78.5270825679419, 78.5334495398768)
(-12.4864543952238, -12.5264337847611)
(-9.31786646179107, 9.37137318645303)
(-56.5309801938186, -56.5398242097896)
(100.521017074687, 100.525991035798)
(-53.3883466217256, 53.3977111400996)
(21.945612879981, -21.9683846624641)
(-15.644128370333, 15.6760566619115)
(-21.945612879981, 21.9683846624641)
(9.31786646179107, -9.37137318645303)
(-34.5285657554621, 34.5430434838806)
(-100.521017074687, -100.525991035798)
(50.2455828375744, 50.255532975858)
(78.5270825679419, -78.5334495398768)
(53.3883466217256, -53.3977111400996)
(56.5309801938186, 56.5398242097896)
(25.0929104121121, 25.1128284538059)
(18.7964043662102, 18.822986402218)
(15.644128370333, -15.6760566619115)
(2.79838604578389, -2.9716938707138)
(47.1026627703624, -47.1132766856486)
(91.0952098694071, -91.1006984668687)
(-72.2427897046973, 72.2497104791231)
(-28.2389365752603, 28.256637077005)
(94.2371684817036, 94.2424740944813)
(69.100567727981, 69.1078031797371)
(34.5285657554621, -34.5430434838806)
(-87.9532251106725, -87.958909766826)
(-97.3791034786112, 97.3842379150654)
(31.3840740178899, 31.400001623573)
(-62.8159348889734, -62.8238941484508)
(-94.2371684817036, -94.2424740944813)
(62.8159348889734, 62.8238941484508)
(-81.6691650818489, -81.6752871140731)
(97.3791034786112, -97.3842379150654)
(28.2389365752603, -28.256637077005)
(75.3849592185347, 75.3915915495896)
(6.12125046689807, 6.20239528557313)
(-37.672573565113, -37.6858434829161)
(81.6691650818489, 81.6752871140731)
(-31.3840740178899, -31.400001623573)
(-40.8162093266346, 40.8284575240806)
(-43.9595528888955, -43.9709255098366)
(-69.100567727981, -69.1078031797371)
(-65.9582857893902, 65.9658659025626)
(40.8162093266346, -40.8284575240806)
(-91.0952098694071, 91.1006984668687)
(-75.3849592185347, -75.3915915495896)
(87.9532251106725, 87.958909766826)
(-2.79838604578389, 2.9716938707138)
(12.4864543952238, 12.5264337847611)
(59.6735041304405, -59.6818824703587)
(37.672573565113, 37.6858434829161)
(84.811211299318, -84.817106541414)
(65.9582857893902, -65.9658659025626)
(-50.2455828375744, -50.255532975858)
Intervals of increase and decrease of the function:Let's find intervals where the function increases and decreases, as well as minima and maxima of the function, for this let's look how the function behaves itself in the extremas and at the slightest deviation from:
Minima of the function at points:
$$x_{1} = -84.811211299318$$
$$x_{2} = -47.1026627703624$$
$$x_{3} = -59.6735041304405$$
$$x_{4} = 43.9595528888955$$
$$x_{5} = -78.5270825679419$$
$$x_{6} = -9.31786646179107$$
$$x_{7} = 100.521017074687$$
$$x_{8} = -53.3883466217256$$
$$x_{9} = -15.644128370333$$
$$x_{10} = -21.945612879981$$
$$x_{11} = -34.5285657554621$$
$$x_{12} = 50.2455828375744$$
$$x_{13} = 56.5309801938186$$
$$x_{14} = 25.0929104121121$$
$$x_{15} = 18.7964043662102$$
$$x_{16} = -72.2427897046973$$
$$x_{17} = -28.2389365752603$$
$$x_{18} = 94.2371684817036$$
$$x_{19} = 69.100567727981$$
$$x_{20} = -97.3791034786112$$
$$x_{21} = 31.3840740178899$$
$$x_{22} = 62.8159348889734$$
$$x_{23} = 75.3849592185347$$
$$x_{24} = 6.12125046689807$$
$$x_{25} = 81.6691650818489$$
$$x_{26} = -40.8162093266346$$
$$x_{27} = -65.9582857893902$$
$$x_{28} = -91.0952098694071$$
$$x_{29} = 87.9532251106725$$
$$x_{30} = -2.79838604578389$$
$$x_{31} = 12.4864543952238$$
$$x_{32} = 37.672573565113$$
Maxima of the function at points:
$$x_{32} = -25.0929104121121$$
$$x_{32} = 72.2427897046973$$
$$x_{32} = -18.7964043662102$$
$$x_{32} = -6.12125046689807$$
$$x_{32} = -12.4864543952238$$
$$x_{32} = -56.5309801938186$$
$$x_{32} = 21.945612879981$$
$$x_{32} = 9.31786646179107$$
$$x_{32} = -100.521017074687$$
$$x_{32} = 78.5270825679419$$
$$x_{32} = 53.3883466217256$$
$$x_{32} = 15.644128370333$$
$$x_{32} = 2.79838604578389$$
$$x_{32} = 47.1026627703624$$
$$x_{32} = 91.0952098694071$$
$$x_{32} = 34.5285657554621$$
$$x_{32} = -87.9532251106725$$
$$x_{32} = -62.8159348889734$$
$$x_{32} = -94.2371684817036$$
$$x_{32} = -81.6691650818489$$
$$x_{32} = 97.3791034786112$$
$$x_{32} = 28.2389365752603$$
$$x_{32} = -37.672573565113$$
$$x_{32} = -31.3840740178899$$
$$x_{32} = -43.9595528888955$$
$$x_{32} = -69.100567727981$$
$$x_{32} = 40.8162093266346$$
$$x_{32} = -75.3849592185347$$
$$x_{32} = 59.6735041304405$$
$$x_{32} = 84.811211299318$$
$$x_{32} = 65.9582857893902$$
$$x_{32} = -50.2455828375744$$
Decreasing at intervals
$$\left[100.521017074687, \infty\right)$$
Increasing at intervals
$$\left(-\infty, -97.3791034786112\right]$$