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$$- 2 \sin{\left(2 x \right)} + \frac{\cos{\left(\frac{x}{3} \right)}}{3} = 0$$
Solve this equationThe roots of this equation
$$x_{1} = 45.6265240874821$$
$$x_{2} = 43.9415690350161$$
$$x_{3} = -56.4649766095654$$
$$x_{4} = -21.948405903593$$
$$x_{5} = -73.9008579697903$$
$$x_{6} = -31.456654651139$$
$$x_{7} = -53.3663469957854$$
$$x_{8} = 70.6858347057703$$
$$x_{9} = 64.4760800090209$$
$$x_{10} = -7.78258244299257$$
$$x_{11} = 34.6002618610233$$
$$x_{12} = 42.4115008234622$$
$$x_{13} = 78.5805444549859$$
$$x_{14} = -83.1808061291476$$
$$x_{15} = -61.261056745001$$
$$x_{16} = -39.3413073608543$$
$$x_{17} = 72.2993737041008$$
$$x_{18} = -50.3062105726777$$
$$x_{19} = -94.1640884526429$$
$$x_{20} = 3.18232076883083$$
$$x_{21} = -67.4708114417504$$
$$x_{22} = 92.605584089917$$
$$x_{23} = 6.24245719193855$$
$$x_{24} = 89.5353906273091$$
$$x_{25} = 23.5619449019235$$
$$x_{26} = -36.2017461267127$$
$$x_{27} = -28.3580250373591$$
$$x_{28} = 65.8897545703347$$
$$x_{29} = 20.3469216379035$$
$$x_{30} = -64.3312502076088$$
$$x_{31} = -88.0053224157552$$
$$x_{32} = -42.4115008234622$$
$$x_{33} = -66.0571368804366$$
$$x_{34} = -72.2159029173242$$
$$x_{35} = 94.3314707627447$$
$$x_{36} = -17.352190205174$$
$$x_{37} = 50.2227397859011$$
$$x_{38} = -37.6154206880266$$
$$x_{39} = 36.0569163253007$$
$$x_{40} = 95.7451453240586$$
$$x_{41} = 73.7560281683782$$
$$x_{42} = -14.1371669411541$$
$$x_{43} = 1.49736571636477$$
$$x_{44} = 48.7660853216237$$
$$x_{45} = -86.3203673632892$$
$$x_{46} = -29.7716995986729$$
$$x_{47} = 7.92741224440461$$
$$x_{48} = 67.6156412431625$$
$$x_{49} = 80.1106126665397$$
$$x_{50} = 26.7769681659434$$
$$x_{51} = -6.32592797871514$$
$$x_{52} = -45.4816942860701$$
$$x_{53} = -1.64219551777681$$
$$x_{54} = -102.030362050686$$
$$x_{55} = -728.890223748073$$
$$x_{56} = 87.9218516289787$$
$$x_{57} = -58.1908632823931$$
$$x_{58} = 58.046033480981$$
$$x_{59} = 51.8362787842316$$
$$x_{60} = -95.8899751254706$$
$$x_{61} = 22.0318766903696$$
$$x_{62} = -9.50846911582031$$
$$x_{63} = 12.5236279428236$$
$$x_{64} = 0.0836911550509267$$
$$x_{65} = -75.3145325311041$$
$$x_{66} = -81.7241516648702$$
$$x_{67} = -20.4917514393156$$
$$x_{68} = 100.490236799632$$
$$x_{69} = 29.916529400085$$
$$x_{70} = 53.449817782562$$
$$x_{71} = -51.8362787842316$$
$$x_{72} = 81.6406808780936$$
$$x_{73} = 4.71238898038469$$
$$x_{74} = 15.7507059394845$$
$$x_{75} = -78.4970736682093$$
$$x_{76} = -23.5619449019235$$
$$x_{77} = -15.6672351527079$$
$$x_{78} = -44.0250398217927$$
$$x_{79} = 59.7309885334471$$
$$x_{80} = -59.6475177466705$$
$$x_{81} = 14.1371669411541$$
$$x_{82} = 86.4651971647012$$
$$x_{83} = -89.5353906273091$$
$$x_{84} = -97.346629589748$$
$$x_{85} = 37.7828029981284$$
$$x_{86} = -703.759497075649$$
$$x_{87} = -48.6212555202117$$
$$x_{88} = -80.1106126665397$$
$$x_{89} = 28.1906427272572$$
$$x_{90} = 56.6323589196672$$
$$x_{91} = -494.800842940392$$
The values of the extrema at the points:
(45.62652408748213, -0.510580506396555)
(43.941569035016066, 1.86941768286213)
(-56.464976609565355, 1.01391768993568)
(-21.948405903593, 0.137534383453571)
(-73.90085796979027, -0.510580506396554)
(-31.456654651138972, 1.86941768286213)
(-53.36634699578545, 1.86941768286214)
(70.68583470577035, -2)
(64.47608000902089, -0.510580506396553)
(-7.782582442992568, -1.51028924985483)
(34.60026186102328, 0.13753438345357)
(42.411500823462205, 0)
(78.58054445498587, 1.86941768286213)
(-83.18080612914761, -1.51028924985483)
(-61.26105674500097, -2)
(-39.34130736085433, -1.51028924985483)
(72.2993737041008, 0.13753438345357)
(-50.30621057267773, 1.86941768286213)
(-94.16408845264287, 1.01391768993568)
(3.182320768830832, 1.86941768286213)
(-67.47081144175043, -0.510580506396555)
(92.60558408991699, -1.51028924985483)
(6.242457191938548, 1.86941768286213)
(89.53539062730911, -2)
(23.56194490192345, 0)
(-36.20174612671275, -0.510580506396554)
(-28.358025037359067, 1.01391768993568)
(65.88975457033473, 1.01391768993568)
(20.34692163790353, -0.510580506396555)
(-64.33125020760885, -1.51028924985483)
(-88.00532241575524, 1.86941768286213)
(-42.411500823462205, -2)
(-66.05713688043659, 1.01391768993568)
(-72.2159029173242, 1.86941768286213)
(94.33147076274473, 1.01391768993568)
(-17.35219020517399, -0.510580506396555)
(50.22273978590114, 0.137534383453572)
(-37.61542068802659, 1.01391768993568)
(36.056916325300705, -1.51028924985483)
(95.74514532405857, -0.510580506396556)
(73.75602816837822, -1.51028924985484)
(-14.137166941154069, 0)
(1.49736571636477, -0.510580506396555)
(48.76608532162371, -1.51028924985483)
(-86.3203673632892, -0.510580506396557)
(-29.77169959867291, -0.510580506396555)
(7.92741224440461, -0.510580506396555)
(67.61564124316247, -1.51028924985483)
(80.11061266653972, 0)
(26.77696816594337, -0.510580506396554)
(-6.32592797871514, 0.137534383453571)
(-45.48169428607009, -1.51028924985483)
(-1.642195517776812, -1.51028924985483)
(-102.03036205068636, -1.51028924985483)
(-728.890223748073, 1.86941768286212)
(87.92185162897866, 0.137534383453572)
(-58.19086328239309, -1.51028924985483)
(58.046033480981045, -0.510580506396557)
(51.83627878423159, -2)
(-95.88997512547061, -1.51028924985483)
(22.031876690369593, 1.86941768286213)
(-9.508469115820306, 1.01391768993568)
(12.52362794282362, 0.137534383453571)
(0.08369115505092671, 1.01391768993568)
(-75.3145325311041, 1.01391768993569)
(-81.72415166487018, 0.137534383453571)
(-20.49175143931557, -1.51028924985483)
(100.49023679963234, 1.86941768286214)
(29.91652940008495, -1.51028924985483)
(53.449817782562036, 0.13753438345357)
(-51.83627878423159, 0)
(81.64068087809359, 1.86941768286213)
(4.71238898038469, 0)
(15.75070593948452, 0.137534383453571)
(-78.49707366820928, 0.137534383453571)
(-23.56194490192345, -2)
(-15.667235152707928, 1.86941768286213)
(-44.02503982179266, 0.137534383453571)
(59.73098853344711, 1.86941768286213)
(-59.647517746670516, 0.137534383453571)
(14.137166941154069, -2)
(86.46519716470122, -1.51028924985483)
(-89.53539062730911, 0)
(-97.34662958974803, 0.137534383453573)
(37.782802998128446, 1.01391768993568)
(-703.7594970756493, 0.137534383453571)
(-48.62125552021167, -0.510580506396555)
(-80.11061266653972, -2)
(28.190642727257213, 1.01391768993568)
(56.632358919667205, 1.01391768993568)
(-494.80084294039244, -2)
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:
$$x_{1} = 45.6265240874821$$
$$x_{2} = -73.9008579697903$$
$$x_{3} = 70.6858347057703$$
$$x_{4} = 64.4760800090209$$
$$x_{5} = -7.78258244299257$$
$$x_{6} = 42.4115008234622$$
$$x_{7} = -83.1808061291476$$
$$x_{8} = -61.261056745001$$
$$x_{9} = -39.3413073608543$$
$$x_{10} = -67.4708114417504$$
$$x_{11} = 92.605584089917$$
$$x_{12} = 89.5353906273091$$
$$x_{13} = 23.5619449019235$$
$$x_{14} = -36.2017461267127$$
$$x_{15} = 20.3469216379035$$
$$x_{16} = -64.3312502076088$$
$$x_{17} = -42.4115008234622$$
$$x_{18} = -17.352190205174$$
$$x_{19} = 36.0569163253007$$
$$x_{20} = 95.7451453240586$$
$$x_{21} = 73.7560281683782$$
$$x_{22} = -14.1371669411541$$
$$x_{23} = 1.49736571636477$$
$$x_{24} = 48.7660853216237$$
$$x_{25} = -86.3203673632892$$
$$x_{26} = -29.7716995986729$$
$$x_{27} = 7.92741224440461$$
$$x_{28} = 67.6156412431625$$
$$x_{29} = 80.1106126665397$$
$$x_{30} = 26.7769681659434$$
$$x_{31} = -45.4816942860701$$
$$x_{32} = -1.64219551777681$$
$$x_{33} = -102.030362050686$$
$$x_{34} = -58.1908632823931$$
$$x_{35} = 58.046033480981$$
$$x_{36} = 51.8362787842316$$
$$x_{37} = -95.8899751254706$$
$$x_{38} = -20.4917514393156$$
$$x_{39} = 29.916529400085$$
$$x_{40} = -51.8362787842316$$
$$x_{41} = 4.71238898038469$$
$$x_{42} = -23.5619449019235$$
$$x_{43} = 14.1371669411541$$
$$x_{44} = 86.4651971647012$$
$$x_{45} = -89.5353906273091$$
$$x_{46} = -48.6212555202117$$
$$x_{47} = -80.1106126665397$$
$$x_{48} = -494.800842940392$$
Maxima of the function at points:
$$x_{48} = 43.9415690350161$$
$$x_{48} = -56.4649766095654$$
$$x_{48} = -21.948405903593$$
$$x_{48} = -31.456654651139$$
$$x_{48} = -53.3663469957854$$
$$x_{48} = 34.6002618610233$$
$$x_{48} = 78.5805444549859$$
$$x_{48} = 72.2993737041008$$
$$x_{48} = -50.3062105726777$$
$$x_{48} = -94.1640884526429$$
$$x_{48} = 3.18232076883083$$
$$x_{48} = 6.24245719193855$$
$$x_{48} = -28.3580250373591$$
$$x_{48} = 65.8897545703347$$
$$x_{48} = -88.0053224157552$$
$$x_{48} = -66.0571368804366$$
$$x_{48} = -72.2159029173242$$
$$x_{48} = 94.3314707627447$$
$$x_{48} = 50.2227397859011$$
$$x_{48} = -37.6154206880266$$
$$x_{48} = -6.32592797871514$$
$$x_{48} = -728.890223748073$$
$$x_{48} = 87.9218516289787$$
$$x_{48} = 22.0318766903696$$
$$x_{48} = -9.50846911582031$$
$$x_{48} = 12.5236279428236$$
$$x_{48} = 0.0836911550509267$$
$$x_{48} = -75.3145325311041$$
$$x_{48} = -81.7241516648702$$
$$x_{48} = 100.490236799632$$
$$x_{48} = 53.449817782562$$
$$x_{48} = 81.6406808780936$$
$$x_{48} = 15.7507059394845$$
$$x_{48} = -78.4970736682093$$
$$x_{48} = -15.6672351527079$$
$$x_{48} = -44.0250398217927$$
$$x_{48} = 59.7309885334471$$
$$x_{48} = -59.6475177466705$$
$$x_{48} = -97.346629589748$$
$$x_{48} = 37.7828029981284$$
$$x_{48} = -703.759497075649$$
$$x_{48} = 28.1906427272572$$
$$x_{48} = 56.6323589196672$$
Decreasing at intervals
$$\left[95.7451453240586, \infty\right)$$
Increasing at intervals
$$\left(-\infty, -494.800842940392\right]$$