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$$\frac{\frac{1}{x \cos{\left(x \right)}} \left(x \sin{\left(x \right)} - \cos{\left(x \right)}\right)}{x \cos{\left(x \right)}} = 0$$
Solve this equationThe roots of this equation
$$x_{1} = -12.6452872238566$$
$$x_{2} = -72.270467060309$$
$$x_{3} = 72.270467060309$$
$$x_{4} = -84.8347887180423$$
$$x_{5} = 9.52933440536196$$
$$x_{6} = 78.5525459842429$$
$$x_{7} = -6.43729817917195$$
$$x_{8} = 37.7256128277765$$
$$x_{9} = -18.90240995686$$
$$x_{10} = -56.5663442798215$$
$$x_{11} = -37.7256128277765$$
$$x_{12} = 22.0364967279386$$
$$x_{13} = -69.1295029738953$$
$$x_{14} = -28.309642854452$$
$$x_{15} = 47.145097736761$$
$$x_{16} = -97.3996388790738$$
$$x_{17} = -25.1724463266467$$
$$x_{18} = -59.7070073053355$$
$$x_{19} = -9.52933440536196$$
$$x_{20} = 6.43729817917195$$
$$x_{21} = -53.4257904773947$$
$$x_{22} = 28.309642854452$$
$$x_{23} = 91.1171613944647$$
$$x_{24} = 25.1724463266467$$
$$x_{25} = -15.7712848748159$$
$$x_{26} = -100.540910786842$$
$$x_{27} = 44.0050179208308$$
$$x_{28} = -65.9885986984904$$
$$x_{29} = 81.6936492356017$$
$$x_{30} = -91.1171613944647$$
$$x_{31} = -47.145097736761$$
$$x_{32} = -3.42561845948173$$
$$x_{33} = -50.2853663377737$$
$$x_{34} = -94.2583883450399$$
$$x_{35} = 97.3996388790738$$
$$x_{36} = -87.9759605524932$$
$$x_{37} = 94.2583883450399$$
$$x_{38} = 75.4114834888481$$
$$x_{39} = 15.7712848748159$$
$$x_{40} = 53.4257904773947$$
$$x_{41} = 69.1295029738953$$
$$x_{42} = -75.4114834888481$$
$$x_{43} = -81.6936492356017$$
$$x_{44} = 3.42561845948173$$
$$x_{45} = 62.8477631944545$$
$$x_{46} = -40.8651703304881$$
$$x_{47} = 59.7070073053355$$
$$x_{48} = -78.5525459842429$$
$$x_{49} = 40.8651703304881$$
$$x_{50} = 12.6452872238566$$
$$x_{51} = -34.5864242152889$$
$$x_{52} = -31.4477146375462$$
$$x_{53} = 34.5864242152889$$
$$x_{54} = -22.0364967279386$$
$$x_{55} = 31.4477146375462$$
$$x_{56} = 50.2853663377737$$
$$x_{57} = 56.5663442798215$$
$$x_{58} = -44.0050179208308$$
$$x_{59} = 100.540910786842$$
$$x_{60} = 65.9885986984904$$
$$x_{61} = -62.8477631944545$$
$$x_{62} = 18.90240995686$$
$$x_{63} = 84.8347887180423$$
$$x_{64} = 87.9759605524932$$
The values of the extrema at the points:
(-12.645287223856643, -0.0793277367906304)
(-72.27046706030896, 0.0138382352591247)
(72.27046706030896, -0.0138382352591247)
(-84.83478871804229, 0.0117884359312579)
(9.529334405361963, -0.105515347800849)
(78.55254598424293, -0.0127313636351895)
(-6.437298179171947, -0.15720788867175)
(37.7256128277765, 0.0265165010956094)
(-18.902409956860023, -0.052977287259947)
(-56.56634427982152, -0.0176811187406714)
(-37.7256128277765, -0.0265165010956094)
(22.036496727938566, -0.0454259641004124)
(-69.12950297389526, -0.0144671171981564)
(-28.30964285445201, 0.0353456838322633)
(47.14509773676103, -0.0212158841190105)
(-97.39963887907376, 0.0102675196288633)
(-25.172446326646664, -0.0397573105340905)
(-59.70700730533546, 0.0167508018010073)
(-9.529334405361963, 0.105515347800849)
(6.437298179171947, 0.15720788867175)
(-53.42579047739466, 0.0187208303161273)
(28.30964285445201, -0.0353456838322633)
(91.11716139446474, -0.0109755418944984)
(25.172446326646664, 0.0397573105340905)
(-15.771284874815882, 0.0635337054506128)
(-100.54091078684232, -0.00994669189285045)
(44.005017920830845, 0.0227305480033923)
(-65.98859869849039, 0.0151558729387687)
(81.69364923560168, 0.0122417706370046)
(-91.11716139446474, 0.0109755418944984)
(-47.14509773676103, 0.0212158841190105)
(-3.4256184594817283, 0.304101903252418)
(-50.28536633777365, -0.0198904331371739)
(-94.25838834503986, -0.0106097323840529)
(97.39963887907376, -0.0102675196288633)
(-87.97596055249322, -0.0113674757631396)
(94.25838834503986, 0.0106097323840529)
(75.41148348884815, 0.0132617457097552)
(15.771284874815882, -0.0635337054506128)
(53.42579047739466, -0.0187208303161273)
(69.12950297389526, 0.0144671171981564)
(-75.41148348884815, -0.0132617457097552)
(-81.69364923560168, -0.0122417706370046)
(3.4256184594817283, -0.304101903252418)
(62.84776319445445, 0.0159134793112296)
(-40.86517033048807, 0.0244780421832437)
(59.70700730533546, -0.0167508018010073)
(-78.55254598424293, 0.0127313636351895)
(40.86517033048807, -0.0244780421832437)
(12.645287223856643, 0.0793277367906304)
(-34.58642421528892, 0.0289251611993725)
(-31.447714637546234, -0.0318148859478787)
(34.58642421528892, -0.0289251611993725)
(-22.036496727938566, 0.0454259641004124)
(31.447714637546234, 0.0318148859478787)
(50.28536633777365, 0.0198904331371739)
(56.56634427982152, 0.0176811187406714)
(-44.005017920830845, -0.0227305480033923)
(100.54091078684232, 0.00994669189285045)
(65.98859869849039, -0.0151558729387687)
(-62.84776319445445, -0.0159134793112296)
(18.902409956860023, 0.052977287259947)
(84.83478871804229, -0.0117884359312579)
(87.97596055249322, 0.0113674757631396)
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:
$$x_{1} = -72.270467060309$$
$$x_{2} = -84.8347887180423$$
$$x_{3} = 37.7256128277765$$
$$x_{4} = -28.309642854452$$
$$x_{5} = -97.3996388790738$$
$$x_{6} = -59.7070073053355$$
$$x_{7} = -9.52933440536196$$
$$x_{8} = 6.43729817917195$$
$$x_{9} = -53.4257904773947$$
$$x_{10} = 25.1724463266467$$
$$x_{11} = -15.7712848748159$$
$$x_{12} = 44.0050179208308$$
$$x_{13} = -65.9885986984904$$
$$x_{14} = 81.6936492356017$$
$$x_{15} = -91.1171613944647$$
$$x_{16} = -47.145097736761$$
$$x_{17} = -3.42561845948173$$
$$x_{18} = 94.2583883450399$$
$$x_{19} = 75.4114834888481$$
$$x_{20} = 69.1295029738953$$
$$x_{21} = 62.8477631944545$$
$$x_{22} = -40.8651703304881$$
$$x_{23} = -78.5525459842429$$
$$x_{24} = 12.6452872238566$$
$$x_{25} = -34.5864242152889$$
$$x_{26} = -22.0364967279386$$
$$x_{27} = 31.4477146375462$$
$$x_{28} = 50.2853663377737$$
$$x_{29} = 56.5663442798215$$
$$x_{30} = 100.540910786842$$
$$x_{31} = 18.90240995686$$
$$x_{32} = 87.9759605524932$$
Maxima of the function at points:
$$x_{32} = -12.6452872238566$$
$$x_{32} = 72.270467060309$$
$$x_{32} = 9.52933440536196$$
$$x_{32} = 78.5525459842429$$
$$x_{32} = -6.43729817917195$$
$$x_{32} = -18.90240995686$$
$$x_{32} = -56.5663442798215$$
$$x_{32} = -37.7256128277765$$
$$x_{32} = 22.0364967279386$$
$$x_{32} = -69.1295029738953$$
$$x_{32} = 47.145097736761$$
$$x_{32} = -25.1724463266467$$
$$x_{32} = 28.309642854452$$
$$x_{32} = 91.1171613944647$$
$$x_{32} = -100.540910786842$$
$$x_{32} = -50.2853663377737$$
$$x_{32} = -94.2583883450399$$
$$x_{32} = 97.3996388790738$$
$$x_{32} = -87.9759605524932$$
$$x_{32} = 15.7712848748159$$
$$x_{32} = 53.4257904773947$$
$$x_{32} = -75.4114834888481$$
$$x_{32} = -81.6936492356017$$
$$x_{32} = 3.42561845948173$$
$$x_{32} = 59.7070073053355$$
$$x_{32} = 40.8651703304881$$
$$x_{32} = -31.4477146375462$$
$$x_{32} = 34.5864242152889$$
$$x_{32} = -44.0050179208308$$
$$x_{32} = 65.9885986984904$$
$$x_{32} = -62.8477631944545$$
$$x_{32} = 84.8347887180423$$
Decreasing at intervals
$$\left[100.540910786842, \infty\right)$$
Increasing at intervals
$$\left(-\infty, -97.3996388790738\right]$$