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$$- 2 x \sin{\left(2 x \right)} + \cos{\left(2 x \right)} = 0$$
Solve this equationThe roots of this equation
$$x_{1} = 36.1352335301545$$
$$x_{2} = -15.7238573187731$$
$$x_{3} = -80.113733189628$$
$$x_{4} = 29.8535036526677$$
$$x_{5} = 42.4173943590211$$
$$x_{6} = -94.2504320905443$$
$$x_{7} = -97.3919391862849$$
$$x_{8} = 64.4065309145547$$
$$x_{9} = -103.674968932212$$
$$x_{10} = 6.32264361192832$$
$$x_{11} = 37.7057417444241$$
$$x_{12} = -86.3966915707365$$
$$x_{13} = -65.9772348386275$$
$$x_{14} = 51.8411010631448$$
$$x_{15} = -43.9879802762466$$
$$x_{16} = 26.7128952386973$$
$$x_{17} = -11.0182483639693$$
$$x_{18} = 95.8211849371972$$
$$x_{19} = 0.43016679450969$$
$$x_{20} = 12.5862231633233$$
$$x_{21} = -39.2762729921215$$
$$x_{22} = 92.6796807176258$$
$$x_{23} = -51.8411010631448$$
$$x_{24} = -59.6944483144154$$
$$x_{25} = -1.71280922974086$$
$$x_{26} = 87.9674362306479$$
$$x_{27} = 81.6844695124177$$
$$x_{28} = 45.5585806972324$$
$$x_{29} = 61.2651372785773$$
$$x_{30} = -22.0025089604154$$
$$x_{31} = 20.432585165244$$
$$x_{32} = 48.6998194395369$$
$$x_{33} = 58.123765151966$$
$$x_{34} = -42.4173943590211$$
$$x_{35} = 94.2504320905443$$
$$x_{36} = -0.43016679450969$$
$$x_{37} = 15.7238573187731$$
$$x_{38} = -36.1352335301545$$
$$x_{39} = 34.5647514869476$$
$$x_{40} = 23.5725488683805$$
$$x_{41} = 65.9772348386275$$
$$x_{42} = -20.432585165244$$
$$x_{43} = 7.88564243740794$$
$$x_{44} = -81.6844695124177$$
$$x_{45} = -9.45120497843001$$
$$x_{46} = 28.2831721399108$$
$$x_{47} = 72.2600907017656$$
$$x_{48} = -23.5725488683805$$
$$x_{49} = 3.21864908958597$$
$$x_{50} = -37.7057417444241$$
$$x_{51} = 1.71280922974086$$
$$x_{52} = -67.5479430595368$$
$$x_{53} = 22.0025089604154$$
$$x_{54} = 70.6893712463639$$
$$x_{55} = 9.45120497843001$$
$$x_{56} = -14.154821427226$$
$$x_{57} = -58.123765151966$$
$$x_{58} = -64.4065309145547$$
$$x_{59} = -89.5381827032021$$
$$x_{60} = -95.8211849371972$$
$$x_{61} = -53.4117555918474$$
$$x_{62} = -6.32264361192832$$
$$x_{63} = -28.2831721399108$$
$$x_{64} = 80.113733189628$$
$$x_{65} = -17.2932121076445$$
$$x_{66} = -50.2704553934212$$
$$x_{67} = -73.8308134276772$$
$$x_{68} = -87.9674362306479$$
$$x_{69} = 89.5381827032021$$
$$x_{70} = -83.2552080991765$$
$$x_{71} = -72.2600907017656$$
$$x_{72} = 114.67031200546$$
$$x_{73} = 14.154821427226$$
$$x_{74} = -61.2651372785773$$
$$x_{75} = 50.2704553934212$$
$$x_{76} = -7.88564243740794$$
$$x_{77} = -45.5585806972324$$
$$x_{78} = 86.3966915707365$$
$$x_{79} = -3.21864908958597$$
$$x_{80} = 59.6944483144154$$
$$x_{81} = -75.4015392197413$$
$$x_{82} = 56.5530882745116$$
$$x_{83} = 73.8308134276772$$
$$x_{84} = -31.4238815972272$$
$$x_{85} = -29.8535036526677$$
$$x_{86} = 100.533451628845$$
$$x_{87} = 43.9879802762466$$
$$x_{88} = 78.542999266617$$
$$x_{89} = 67.5479430595368$$
The values of the extrema at the points:
(36.13523353015448, -36.1317747991247)
(-15.723857318773117, -15.7159136392673)
(-80.11373318962796, 80.112172953406)
(29.85350365266773, -29.8493174201329)
(42.417394359021145, -42.4144477618284)
(-94.25043209054431, -94.2491058646707)
(-97.39193918628494, -97.390655737879)
(64.40653091455466, -64.4045902053056)
(-103.67496893221228, -103.673763262022)
(6.322643611928322, 6.30296564894634)
(37.705741744424074, 37.702427036601)
(-86.39669157073652, 86.3952447924177)
(-65.97723483862752, -65.9753403273413)
(51.84110106314479, -51.8386900171372)
(-43.98798027624661, -43.9851388662124)
(26.71289523869733, -26.7082170799481)
(-11.018248363969283, 11.0069210395792)
(95.82118493719717, -95.8198804506423)
(0.43016679450968986, 0.280548169095523)
(12.586223163323332, 12.5763034089358)
(-39.27627299212146, 39.2730907958671)
(92.67968071762581, -92.6783320156182)
(-51.84110106314479, 51.8386900171372)
(-59.69444831441541, -59.6923544275184)
(-1.7128092297408641, 1.64418569779545)
(87.96743623064788, 87.9660152847086)
(81.68446951241769, 81.6829392767655)
(45.55858069723237, -45.5558372248235)
(61.2651372785773, -61.263097068409)
(-22.002508960415422, -21.9968299895532)
(20.432585165244035, -20.4264702322587)
(48.69981943953688, -48.6972528978117)
(58.12376515196605, -58.1216146879934)
(-42.417394359021145, 42.4144477618284)
(94.25043209054431, 94.2491058646707)
(-0.43016679450968986, -0.280548169095523)
(15.723857318773117, 15.7159136392673)
(-36.13523353015448, 36.1317747991247)
(34.56475148694763, 34.5611356534609)
(23.572548868380515, -23.567247878771)
(65.97723483862752, 65.9753403273413)
(-20.432585165244035, 20.4264702322587)
(7.885642437407941, -7.86983848106687)
(-81.68446951241769, -81.6829392767655)
(-9.451204978430011, -9.43800684898451)
(28.28317213991076, 28.2787535864381)
(72.26009070176562, 72.2583609016736)
(-23.572548868380515, 23.567247878771)
(3.2186490895859734, 3.18050197241693)
(-37.705741744424074, -37.702427036601)
(1.7128092297408641, -1.64418569779545)
(-67.54794305953683, 67.546092598104)
(22.002508960415422, 21.9968299895532)
(70.68937124636392, -70.687603012927)
(9.451204978430011, 9.43800684898451)
(-14.154821427226006, 14.1459987695472)
(-58.12376515196605, 58.1216146879934)
(-64.40653091455466, 64.4045902053056)
(-89.53818270320214, 89.5367866833941)
(-95.82118493719717, 95.8198804506423)
(-53.41175559184737, -53.4094154368825)
(-6.322643611928322, -6.30296564894634)
(-28.28317213991076, -28.2787535864381)
(80.11373318962796, -80.112172953406)
(-17.29321210764446, 17.2859883667942)
(-50.27045539342116, -50.267969027913)
(-73.83081342767719, 73.829120425871)
(-87.96743623064788, -87.9660152847086)
(89.53818270320214, -89.5367866833941)
(-83.2552080991765, 83.2537067322156)
(-72.26009070176562, -72.2583609016736)
(114.67031200545999, -114.669221939379)
(14.154821427226006, -14.1459987695472)
(-61.2651372785773, 61.263097068409)
(50.27045539342116, 50.267969027913)
(-7.885642437407941, 7.86983848106687)
(-45.55858069723237, 45.5558372248235)
(86.39669157073652, -86.3952447924177)
(-3.2186490895859734, -3.18050197241693)
(59.69444831441541, 59.6923544275184)
(-75.40153921974125, -75.3998814833205)
(56.55308827451163, 56.5508780915478)
(73.83081342767719, -73.829120425871)
(-31.423881597227226, -31.4199044860773)
(-29.85350365266773, 29.8493174201329)
(100.53345162884467, 100.532208284673)
(43.98798027624661, 43.9851388662124)
(78.54299926661696, 78.5414078300528)
(67.54794305953683, -67.546092598104)
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:
$$x_{1} = 36.1352335301545$$
$$x_{2} = -15.7238573187731$$
$$x_{3} = 29.8535036526677$$
$$x_{4} = 42.4173943590211$$
$$x_{5} = -94.2504320905443$$
$$x_{6} = -97.3919391862849$$
$$x_{7} = 64.4065309145547$$
$$x_{8} = -103.674968932212$$
$$x_{9} = -65.9772348386275$$
$$x_{10} = 51.8411010631448$$
$$x_{11} = -43.9879802762466$$
$$x_{12} = 26.7128952386973$$
$$x_{13} = 95.8211849371972$$
$$x_{14} = 92.6796807176258$$
$$x_{15} = -59.6944483144154$$
$$x_{16} = 45.5585806972324$$
$$x_{17} = 61.2651372785773$$
$$x_{18} = -22.0025089604154$$
$$x_{19} = 20.432585165244$$
$$x_{20} = 48.6998194395369$$
$$x_{21} = 58.123765151966$$
$$x_{22} = -0.43016679450969$$
$$x_{23} = 23.5725488683805$$
$$x_{24} = 7.88564243740794$$
$$x_{25} = -81.6844695124177$$
$$x_{26} = -9.45120497843001$$
$$x_{27} = -37.7057417444241$$
$$x_{28} = 1.71280922974086$$
$$x_{29} = 70.6893712463639$$
$$x_{30} = -53.4117555918474$$
$$x_{31} = -6.32264361192832$$
$$x_{32} = -28.2831721399108$$
$$x_{33} = 80.113733189628$$
$$x_{34} = -50.2704553934212$$
$$x_{35} = -87.9674362306479$$
$$x_{36} = 89.5381827032021$$
$$x_{37} = -72.2600907017656$$
$$x_{38} = 114.67031200546$$
$$x_{39} = 14.154821427226$$
$$x_{40} = 86.3966915707365$$
$$x_{41} = -3.21864908958597$$
$$x_{42} = -75.4015392197413$$
$$x_{43} = 73.8308134276772$$
$$x_{44} = -31.4238815972272$$
$$x_{45} = 67.5479430595368$$
Maxima of the function at points:
$$x_{45} = -80.113733189628$$
$$x_{45} = 6.32264361192832$$
$$x_{45} = 37.7057417444241$$
$$x_{45} = -86.3966915707365$$
$$x_{45} = -11.0182483639693$$
$$x_{45} = 0.43016679450969$$
$$x_{45} = 12.5862231633233$$
$$x_{45} = -39.2762729921215$$
$$x_{45} = -51.8411010631448$$
$$x_{45} = -1.71280922974086$$
$$x_{45} = 87.9674362306479$$
$$x_{45} = 81.6844695124177$$
$$x_{45} = -42.4173943590211$$
$$x_{45} = 94.2504320905443$$
$$x_{45} = 15.7238573187731$$
$$x_{45} = -36.1352335301545$$
$$x_{45} = 34.5647514869476$$
$$x_{45} = 65.9772348386275$$
$$x_{45} = -20.432585165244$$
$$x_{45} = 28.2831721399108$$
$$x_{45} = 72.2600907017656$$
$$x_{45} = -23.5725488683805$$
$$x_{45} = 3.21864908958597$$
$$x_{45} = -67.5479430595368$$
$$x_{45} = 22.0025089604154$$
$$x_{45} = 9.45120497843001$$
$$x_{45} = -14.154821427226$$
$$x_{45} = -58.123765151966$$
$$x_{45} = -64.4065309145547$$
$$x_{45} = -89.5381827032021$$
$$x_{45} = -95.8211849371972$$
$$x_{45} = -17.2932121076445$$
$$x_{45} = -73.8308134276772$$
$$x_{45} = -83.2552080991765$$
$$x_{45} = -61.2651372785773$$
$$x_{45} = 50.2704553934212$$
$$x_{45} = -7.88564243740794$$
$$x_{45} = -45.5585806972324$$
$$x_{45} = 59.6944483144154$$
$$x_{45} = 56.5530882745116$$
$$x_{45} = -29.8535036526677$$
$$x_{45} = 100.533451628845$$
$$x_{45} = 43.9879802762466$$
$$x_{45} = 78.542999266617$$
Decreasing at intervals
$$\left[114.67031200546, \infty\right)$$
Increasing at intervals
$$\left(-\infty, -103.674968932212\right]$$