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$$6 x \cos{\left(x x \right)} + \frac{3}{10} = 0$$
Solve this equationThe roots of this equation
$$x_{1} = 70.1296127625783$$
$$x_{2} = -74.874370932554$$
$$x_{3} = 39.8112389867878$$
$$x_{4} = -75.9367761635072$$
$$x_{5} = -31.5327209293145$$
$$x_{6} = 62.2255072223482$$
$$x_{7} = 11.9556799728636$$
$$x_{8} = 8.7735237509369$$
$$x_{9} = 54.1251888981177$$
$$x_{10} = -77.1068313994789$$
$$x_{11} = -97.7504709318572$$
$$x_{12} = 96.097404152879$$
$$x_{13} = 94.2155481437112$$
$$x_{14} = -81.7925556950147$$
$$x_{15} = 10.2585807940195$$
$$x_{16} = -42.5204391798376$$
$$x_{17} = 18.2915923611646$$
$$x_{18} = -3.75817272230474$$
$$x_{19} = 4.15532508372674$$
$$x_{20} = -0.0500001562523601$$
$$x_{21} = 42.0000367204035$$
$$x_{22} = 68.1761365698555$$
$$x_{23} = -93.3109027674001$$
$$x_{24} = 93.6302025729842$$
$$x_{25} = -7.82735233936065$$
$$x_{26} = 45.6901456841463$$
$$x_{27} = 18.1190284461804$$
$$x_{28} = 80.182728745954$$
$$x_{29} = -33.7930526761713$$
$$x_{30} = 81.1176086268132$$
$$x_{31} = 22.3146740122704$$
$$x_{32} = -29.8961776401417$$
$$x_{33} = -85.7673584841745$$
$$x_{34} = 22.802004108798$$
$$x_{35} = -14.2347888508383$$
$$x_{36} = 20.2479705323586$$
$$x_{37} = 76.9640974655712$$
$$x_{38} = -99.0276828623687$$
$$x_{39} = 30.7254002369791$$
$$x_{40} = -71.7460721294522$$
$$x_{41} = -83.8971855570874$$
$$x_{42} = -21.7441347239335$$
$$x_{43} = -69.0006115273383$$
$$x_{44} = -1.23708025803222$$
$$x_{45} = 6.26720721611652$$
$$x_{46} = 84.2521691550406$$
$$x_{47} = 46.0326550508655$$
$$x_{48} = -57.7477399351734$$
$$x_{49} = 2.1654785484363$$
$$x_{50} = 60.2503898854834$$
$$x_{51} = 1.26894030078895$$
$$x_{52} = 98.5028396454218$$
$$x_{53} = 5.16848253607869$$
$$x_{54} = -47.8726736795755$$
$$x_{55} = 83.2015553414404$$
$$x_{56} = -33.3720406543686$$
$$x_{57} = -2.79930689990574$$
$$x_{58} = 5.46223180703294$$
$$x_{59} = 32.1249862743282$$
$$x_{60} = -69.7703270264263$$
$$x_{61} = 92.1079339897528$$
$$x_{62} = 56.119938757743$$
$$x_{63} = 16.0011460991491$$
$$x_{64} = -65.8556595824873$$
$$x_{65} = -16.0013413816057$$
$$x_{66} = 63.6728050640327$$
$$x_{67} = 66.4492868446822$$
$$x_{68} = 52.4149189591217$$
$$x_{69} = -43.9733034389992$$
$$x_{70} = 58.1813133648635$$
$$x_{71} = 14.0126053634841$$
$$x_{72} = 34.2088398673973$$
$$x_{73} = -17.8570561503462$$
$$x_{74} = -93.7978128386623$$
$$x_{75} = 36.0422970272423$$
$$x_{76} = 46.9783982641596$$
$$x_{77} = 71.6803605418173$$
$$x_{78} = -9.12396433677114$$
$$x_{79} = 54.4434729814154$$
$$x_{80} = -53.6295534820434$$
$$x_{81} = 27.99688520455$$
$$x_{82} = 82.2330760392673$$
$$x_{83} = 40.164743030986$$
The values of the extrema at the points:
(70.12961276257828, 18.0388845912535)
(-74.87437093255404, -19.462311948672)
(39.81123898678778, 14.9433693300074)
(-75.93677616350725, -25.7810321987323)
(-31.532720929314547, -6.4598200502469)
(62.22550722234824, 21.667651198216)
(11.955679972863633, 0.586730227072654)
(8.7735237509369, 5.63200840759171)
(54.125188898117656, 19.2375553893689)
(-77.10683139947889, -20.1320500505767)
(-97.75047093185724, -32.3251408870988)
(96.09740415287895, 25.8292216519403)
(94.21554814371122, 25.2646648655739)
(-81.79255569501468, -27.5377661479681)
(10.258580794019508, 0.0776098717713922)
(-42.52043917983758, -15.7561296798215)
(18.291592361164593, 8.48746650032543)
(-3.7581727223047365, 1.87228266344572)
(4.155325083726742, -1.75318528630622)
(-0.050000156252360074, -0.0075000078125708)
(42.00003672040345, 9.60001314196841)
(68.17613656985549, 17.4528417777579)
(-93.31090276740012, -30.9932703995283)
(93.63020257298423, 31.0890603441361)
(-7.82735233936065, -5.34814449412642)
(45.69014568414627, 16.7070419089131)
(18.119028446180383, 8.43569711132481)
(80.182728745954, 27.0548180405162)
(-33.793052676171314, -13.1379125190517)
(81.11760862681321, 27.3352820181408)
(22.314674012270398, 9.69439467271456)
(-29.89617764014173, -5.96885748770207)
(-85.76735848417451, -28.7302070354671)
(22.802004108798037, 3.84060844513816)
(-14.234788850838335, -1.27045516201812)
(20.247970532358625, 9.07438201291293)
(76.96409746557123, 20.0892298727461)
(-99.02768286236875, -32.7083044763105)
(30.725400236979066, 12.2176160988448)
(-71.7460721294522, -18.5238223673449)
(-83.89718555708735, -22.1691561998922)
(-21.744134723933474, -3.52324834853856)
(-69.00061152733832, -23.7001826705657)
(-1.2370802580322207, 2.62642452932163)
(6.267207216116521, 4.88006668974637)
(84.25216915504059, 22.2756512747982)
(46.032655050865536, 16.8097947455609)
(-57.74773993517343, -20.3243208560471)
(2.1654785484363024, -2.34955663548487)
(60.250389885483415, 15.0751179986719)
(1.268940300788954, 3.37835229605937)
(98.50283964542179, 32.5508515071405)
(5.1684825360786855, 4.55040437756381)
(-47.87267367957547, -17.3618004675987)
(83.2015553414404, 21.9604671441441)
(-33.3720406543686, -7.01161556348787)
(-2.7993068999057353, 2.15972933863806)
(5.462231807032936, -1.36120476806793)
(32.12498627432818, 12.6374922486272)
(-69.77032702642633, -23.9310973375748)
(92.10793398975284, 30.6323797549106)
(56.119938757743014, 19.8359804366376)
(16.0011460991491, 1.80035847611964)
(-65.85565958248728, -16.7566987394057)
(-16.001341381605684, -7.80038776846429)
(63.67280506403273, 22.1018405942489)
(66.44928684468223, 16.9347869026842)
(52.414918959121714, 18.7244743227715)
(-43.97330343899915, -16.191989092363)
(58.18131336486346, 14.4543951172666)
(14.012605363484129, 7.20376251073835)
(34.20883986739731, 13.2626487557595)
(-17.85705615034624, -8.35710508496662)
(-93.7978128386623, -25.1393442778305)
(36.04229702724232, 7.81269199490528)
(46.97839826415957, 17.0935177800851)
(71.68036054181728, 18.5041088923907)
(-9.123964336771142, 0.262765651813411)
(54.44347298141538, 13.3330431595679)
(-53.62955348204343, -19.0888647407769)
(27.99688520454996, 5.39907034559643)
(82.23307603926726, 27.6699222572333)
(40.164743030986024, 9.0494252338595)
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:
$$x_{1} = 70.1296127625783$$
$$x_{2} = -75.9367761635072$$
$$x_{3} = 11.9556799728636$$
$$x_{4} = -97.7504709318572$$
$$x_{5} = 96.097404152879$$
$$x_{6} = 94.2155481437112$$
$$x_{7} = -81.7925556950147$$
$$x_{8} = 10.2585807940195$$
$$x_{9} = -42.5204391798376$$
$$x_{10} = 4.15532508372674$$
$$x_{11} = -0.0500001562523601$$
$$x_{12} = 42.0000367204035$$
$$x_{13} = 68.1761365698555$$
$$x_{14} = -93.3109027674001$$
$$x_{15} = -7.82735233936065$$
$$x_{16} = -33.7930526761713$$
$$x_{17} = -85.7673584841745$$
$$x_{18} = 22.802004108798$$
$$x_{19} = 76.9640974655712$$
$$x_{20} = -99.0276828623687$$
$$x_{21} = -69.0006115273383$$
$$x_{22} = 84.2521691550406$$
$$x_{23} = -57.7477399351734$$
$$x_{24} = 2.1654785484363$$
$$x_{25} = 60.2503898854834$$
$$x_{26} = -47.8726736795755$$
$$x_{27} = 83.2015553414404$$
$$x_{28} = 5.46223180703294$$
$$x_{29} = -69.7703270264263$$
$$x_{30} = 16.0011460991491$$
$$x_{31} = -16.0013413816057$$
$$x_{32} = 66.4492868446822$$
$$x_{33} = -43.9733034389992$$
$$x_{34} = 58.1813133648635$$
$$x_{35} = -17.8570561503462$$
$$x_{36} = 36.0422970272423$$
$$x_{37} = 71.6803605418173$$
$$x_{38} = 54.4434729814154$$
$$x_{39} = -53.6295534820434$$
$$x_{40} = 27.99688520455$$
$$x_{41} = 40.164743030986$$
Maxima of the function at points:
$$x_{41} = -74.874370932554$$
$$x_{41} = 39.8112389867878$$
$$x_{41} = -31.5327209293145$$
$$x_{41} = 62.2255072223482$$
$$x_{41} = 8.7735237509369$$
$$x_{41} = 54.1251888981177$$
$$x_{41} = -77.1068313994789$$
$$x_{41} = 18.2915923611646$$
$$x_{41} = -3.75817272230474$$
$$x_{41} = 93.6302025729842$$
$$x_{41} = 45.6901456841463$$
$$x_{41} = 18.1190284461804$$
$$x_{41} = 80.182728745954$$
$$x_{41} = 81.1176086268132$$
$$x_{41} = 22.3146740122704$$
$$x_{41} = -29.8961776401417$$
$$x_{41} = -14.2347888508383$$
$$x_{41} = 20.2479705323586$$
$$x_{41} = 30.7254002369791$$
$$x_{41} = -71.7460721294522$$
$$x_{41} = -83.8971855570874$$
$$x_{41} = -21.7441347239335$$
$$x_{41} = -1.23708025803222$$
$$x_{41} = 6.26720721611652$$
$$x_{41} = 46.0326550508655$$
$$x_{41} = 1.26894030078895$$
$$x_{41} = 98.5028396454218$$
$$x_{41} = 5.16848253607869$$
$$x_{41} = -33.3720406543686$$
$$x_{41} = -2.79930689990574$$
$$x_{41} = 32.1249862743282$$
$$x_{41} = 92.1079339897528$$
$$x_{41} = 56.119938757743$$
$$x_{41} = -65.8556595824873$$
$$x_{41} = 63.6728050640327$$
$$x_{41} = 52.4149189591217$$
$$x_{41} = 14.0126053634841$$
$$x_{41} = 34.2088398673973$$
$$x_{41} = -93.7978128386623$$
$$x_{41} = 46.9783982641596$$
$$x_{41} = -9.12396433677114$$
$$x_{41} = 82.2330760392673$$
Decreasing at intervals
$$\left[96.097404152879, \infty\right)$$
Increasing at intervals
$$\left(-\infty, -99.0276828623687\right]$$