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$$x \sin{\left(x \right)} \cos{\left(x \right)} - \frac{\cos^{2}{\left(x \right)}}{2} = 0$$
Solve this equationThe roots of this equation
$$x_{1} = 86.3937979737193$$
$$x_{2} = 92.6769832808989$$
$$x_{3} = -7.85398163397448$$
$$x_{4} = 1.5707963267949$$
$$x_{5} = 34.5719807601687$$
$$x_{6} = -64.4026493985908$$
$$x_{7} = -58.1194640914112$$
$$x_{8} = -20.4203522483337$$
$$x_{9} = -45.553093477052$$
$$x_{10} = 64.4026493985908$$
$$x_{11} = -29.845130209103$$
$$x_{12} = -59.6986356231676$$
$$x_{13} = -9.4774857054208$$
$$x_{14} = 70.6858347057703$$
$$x_{15} = 28.2920048800691$$
$$x_{16} = 72.26355003974$$
$$x_{17} = -97.3945059759883$$
$$x_{18} = 42.4115008234622$$
$$x_{19} = -61.261056745001$$
$$x_{20} = 87.970277977177$$
$$x_{21} = -3.29231002128209$$
$$x_{22} = 95.8185759344887$$
$$x_{23} = 58.1194640914112$$
$$x_{24} = -73.8274273593601$$
$$x_{25} = 7.85398163397448$$
$$x_{26} = -50.2754273458806$$
$$x_{27} = -31.43183263459$$
$$x_{28} = -72.26355003974$$
$$x_{29} = 89.5353906273091$$
$$x_{30} = 81.6875298021918$$
$$x_{31} = 23.5619449019235$$
$$x_{32} = 15.7397193560049$$
$$x_{33} = -75.4048544617952$$
$$x_{34} = 3.29231002128209$$
$$x_{35} = -28.2920048800691$$
$$x_{36} = -43.9936619344429$$
$$x_{37} = -42.4115008234622$$
$$x_{38} = -67.5442420521806$$
$$x_{39} = -102.101761241668$$
$$x_{40} = 20.4203522483337$$
$$x_{41} = -37.7123693157661$$
$$x_{42} = -14.1371669411541$$
$$x_{43} = 9.4774857054208$$
$$x_{44} = 22.013857636623$$
$$x_{45} = 59.6986356231676$$
$$x_{46} = 29.845130209103$$
$$x_{47} = 6.36162039206566$$
$$x_{48} = 26.7035375555132$$
$$x_{49} = 48.6946861306418$$
$$x_{50} = 43.9936619344429$$
$$x_{51} = -86.3937979737193$$
$$x_{52} = -94.253084424113$$
$$x_{53} = 78.5461819355535$$
$$x_{54} = 37.7123693157661$$
$$x_{55} = -84.8288957966139$$
$$x_{56} = -22.013857636623$$
$$x_{57} = 80.1106126665397$$
$$x_{58} = -95.8185759344887$$
$$x_{59} = -36.1283155162826$$
$$x_{60} = 51.8362787842316$$
$$x_{61} = -6.36162039206566$$
$$x_{62} = -81.6875298021918$$
$$x_{63} = 100.535938219808$$
$$x_{64} = -1.5707963267949$$
$$x_{65} = 94.253084424113$$
$$x_{66} = -83.2522053201295$$
$$x_{67} = -17.2787595947439$$
$$x_{68} = -65.9810235167388$$
$$x_{69} = 45.553093477052$$
$$x_{70} = -12.6060134442754$$
$$x_{71} = -51.8362787842316$$
$$x_{72} = -15.7397193560049$$
$$x_{73} = -39.2699081698724$$
$$x_{74} = 4.71238898038469$$
$$x_{75} = 36.1283155162826$$
$$x_{76} = 50.2754273458806$$
$$x_{77} = 56.5575080935408$$
$$x_{78} = 12.6060134442754$$
$$x_{79} = -23.5619449019235$$
$$x_{80} = -53.4164352526291$$
$$x_{81} = -0.653271187094403$$
$$x_{82} = 73.8274273593601$$
$$x_{83} = 14.1371669411541$$
$$x_{84} = -89.5353906273091$$
$$x_{85} = 65.9810235167388$$
$$x_{86} = -80.1106126665397$$
$$x_{87} = -87.970277977177$$
$$x_{88} = 67.5442420521806$$
The values of the extrema at the points:
(86.39379797371932, 1.5)
(92.6769832808989, 1.5)
(-7.853981633974483, 1.5)
(1.5707963267948966, 1.5)
(34.57198076016866, -15.782375491468)
(-64.40264939859077, 1.5)
(-58.119464091411174, 1.5)
(-20.420352248333657, 1.5)
(-45.553093477052, 1.5)
(64.40264939859077, 1.5)
(-29.845130209103036, 1.5)
(-59.698635623167625, 31.3472241082539)
(-9.477485705420795, 6.22559030761139)
(70.68583470577035, 1.5)
(28.292004880069126, -12.6415856102068)
(72.26355003974, -34.630045323281)
(-97.39450597598831, 50.1959695818677)
(42.411500823462205, 1.5)
(-61.26105674500097, 1.5)
(87.970277977177, -42.4837181000237)
(-3.2923100212820864, 3.10904369100389)
(95.81857593448869, 1.5)
(58.119464091411174, 1.5)
(-73.82742735936014, 1.5)
(7.853981633974483, 1.5)
(-50.27542734588058, 26.6352276147523)
(-31.431832634590037, 17.2119404633058)
(-72.26355003974, 37.630045323281)
(89.53539062730911, 1.5)
(81.6875298021918, -39.3422347371)
(23.56194490192345, 1.5)
(15.73971935600487, -6.36192599231195)
(-75.40485446179518, 39.2007695855766)
(3.2923100212820864, -0.109043691003894)
(-28.292004880069126, 15.6415856102068)
(-43.993661934442905, 23.4939900158114)
(-42.411500823462205, 1.5)
(-67.54424205218055, 1.5)
(-102.10176124166829, 1.5)
(20.420352248333657, 1.5)
(-37.712369315766125, 20.3528706780541)
(-14.137166941154069, 1.5)
(9.477485705420795, -3.22559030761139)
(22.013857636622962, -9.50125350458599)
(59.698635623167625, -28.3472241082539)
(29.845130209103036, 1.5)
(6.361620392065665, -1.6612817488405)
(26.703537555513243, 1.5)
(48.6946861306418, 1.5)
(43.993661934442905, -20.4939900158114)
(-86.39379797371932, 1.5)
(-94.25308442411298, 48.6252160328321)
(78.54618193555346, -37.7714996118141)
(37.712369315766125, -17.3528706780541)
(-84.8288957966139, 43.9129743950125)
(-22.013857636622962, 12.501253504586)
(80.11061266653972, 1.5)
(-95.81857593448869, 1.5)
(-36.12831551628262, 1.5)
(51.83627878423159, 1.5)
(-6.361620392065665, 4.6612817488405)
(-81.6875298021918, 42.3422347371)
(100.53593821980844, -48.7667258041718)
(-1.5707963267948966, 1.5)
(94.25308442411298, -45.6252160328321)
(-83.25220532012952, 1.5)
(-17.278759594743864, 1.5)
(-65.9810235167388, 34.4886173830534)
(45.553093477052, 1.5)
(-12.606013444275414, 7.79310639486991)
(-51.83627878423159, 1.5)
(-15.73971935600487, 9.36192599231195)
(-39.269908169872416, 1.5)
(4.71238898038469, 1.5)
(36.12831551628262, 1.5)
(50.27542734588058, -23.6352276147523)
(56.55750809354077, -26.7765440796848)
(12.606013444275414, -4.79310639486991)
(-23.56194490192345, 1.5)
(-53.41643525262913, 28.2058777275887)
(-0.6532711870944031, 1.70597463992079)
(73.82742735936014, 1.5)
(14.137166941154069, 1.5)
(-89.53539062730911, 1.5)
(65.9810235167388, -31.4886173830534)
(-80.11061266653972, 1.5)
(-87.970277977177, 45.4837181000237)
(67.54424205218055, 1.5)
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:
$$x_{1} = -7.85398163397448$$
$$x_{2} = 34.5719807601687$$
$$x_{3} = -64.4026493985908$$
$$x_{4} = -58.1194640914112$$
$$x_{5} = -20.4203522483337$$
$$x_{6} = -45.553093477052$$
$$x_{7} = -29.845130209103$$
$$x_{8} = 28.2920048800691$$
$$x_{9} = 72.26355003974$$
$$x_{10} = -61.261056745001$$
$$x_{11} = 87.970277977177$$
$$x_{12} = -73.8274273593601$$
$$x_{13} = 81.6875298021918$$
$$x_{14} = 15.7397193560049$$
$$x_{15} = 3.29231002128209$$
$$x_{16} = -42.4115008234622$$
$$x_{17} = -67.5442420521806$$
$$x_{18} = -102.101761241668$$
$$x_{19} = -14.1371669411541$$
$$x_{20} = 9.4774857054208$$
$$x_{21} = 22.013857636623$$
$$x_{22} = 59.6986356231676$$
$$x_{23} = 6.36162039206566$$
$$x_{24} = 43.9936619344429$$
$$x_{25} = -86.3937979737193$$
$$x_{26} = 78.5461819355535$$
$$x_{27} = 37.7123693157661$$
$$x_{28} = -95.8185759344887$$
$$x_{29} = -36.1283155162826$$
$$x_{30} = 100.535938219808$$
$$x_{31} = -1.5707963267949$$
$$x_{32} = 94.253084424113$$
$$x_{33} = -83.2522053201295$$
$$x_{34} = -17.2787595947439$$
$$x_{35} = -51.8362787842316$$
$$x_{36} = -39.2699081698724$$
$$x_{37} = 50.2754273458806$$
$$x_{38} = 56.5575080935408$$
$$x_{39} = 12.6060134442754$$
$$x_{40} = -23.5619449019235$$
$$x_{41} = -89.5353906273091$$
$$x_{42} = 65.9810235167388$$
$$x_{43} = -80.1106126665397$$
Maxima of the function at points:
$$x_{43} = 86.3937979737193$$
$$x_{43} = 92.6769832808989$$
$$x_{43} = 1.5707963267949$$
$$x_{43} = 64.4026493985908$$
$$x_{43} = -59.6986356231676$$
$$x_{43} = -9.4774857054208$$
$$x_{43} = 70.6858347057703$$
$$x_{43} = -97.3945059759883$$
$$x_{43} = 42.4115008234622$$
$$x_{43} = -3.29231002128209$$
$$x_{43} = 95.8185759344887$$
$$x_{43} = 58.1194640914112$$
$$x_{43} = 7.85398163397448$$
$$x_{43} = -50.2754273458806$$
$$x_{43} = -31.43183263459$$
$$x_{43} = -72.26355003974$$
$$x_{43} = 89.5353906273091$$
$$x_{43} = 23.5619449019235$$
$$x_{43} = -75.4048544617952$$
$$x_{43} = -28.2920048800691$$
$$x_{43} = -43.9936619344429$$
$$x_{43} = 20.4203522483337$$
$$x_{43} = -37.7123693157661$$
$$x_{43} = 29.845130209103$$
$$x_{43} = 26.7035375555132$$
$$x_{43} = 48.6946861306418$$
$$x_{43} = -94.253084424113$$
$$x_{43} = -84.8288957966139$$
$$x_{43} = -22.013857636623$$
$$x_{43} = 80.1106126665397$$
$$x_{43} = 51.8362787842316$$
$$x_{43} = -6.36162039206566$$
$$x_{43} = -81.6875298021918$$
$$x_{43} = -65.9810235167388$$
$$x_{43} = 45.553093477052$$
$$x_{43} = -12.6060134442754$$
$$x_{43} = -15.7397193560049$$
$$x_{43} = 4.71238898038469$$
$$x_{43} = 36.1283155162826$$
$$x_{43} = -53.4164352526291$$
$$x_{43} = -0.653271187094403$$
$$x_{43} = 73.8274273593601$$
$$x_{43} = 14.1371669411541$$
$$x_{43} = -87.970277977177$$
$$x_{43} = 67.5442420521806$$
Decreasing at intervals
$$\left[100.535938219808, \infty\right)$$
Increasing at intervals
$$\left(-\infty, -102.101761241668\right]$$