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$$2 \sin{\left(x \right)} \cos{\left(x \right)} - \frac{\tan^{2}{\left(\frac{x}{3} \right)}}{3} - \frac{1}{3} = 0$$
Solve this equationThe roots of this equation
$$x_{1} = 35.0465923741037$$
$$x_{2} = -28.1038431485559$$
$$x_{3} = -87.4755211158983$$
$$x_{4} = -96.0782214121364$$
$$x_{5} = 95.6061285845669$$
$$x_{6} = -9.25428722701718$$
$$x_{7} = -944.308237881381$$
$$x_{8} = -94.0772888739416$$
$$x_{9} = 53.8961482956424$$
$$x_{10} = -2.65251946897385$$
$$x_{11} = 35.8686700386349$$
$$x_{12} = -37.5286211093253$$
$$x_{13} = -39.5295536475201$$
$$x_{14} = -74.0398747092819$$
$$x_{15} = -49.7764092728208$$
$$x_{16} = 92.4173378032512$$
$$x_{17} = -45.7655408269738$$
$$x_{18} = 67.3317947022587$$
$$x_{19} = -21.5020753905126$$
$$x_{20} = 56.7191584983685$$
$$x_{21} = -58.3791095690589$$
$$x_{22} = -56.3781770308641$$
$$x_{23} = 63.3209262564118$$
$$x_{24} = -96.9002990766677$$
$$x_{25} = 7.59433615632679$$
$$x_{26} = 8180.87776068157$$
$$x_{27} = 10.7831269376425$$
$$x_{28} = 0.170490733752203$$
$$x_{29} = 82.1704821779506$$
$$x_{30} = 28.4448246160603$$
$$x_{31} = -30.1047756867507$$
$$x_{32} = 9.59526869452158$$
$$x_{33} = 6.77225849179553$$
$$x_{34} = -111.738986552359$$
$$x_{35} = -83.4646526700513$$
$$x_{36} = 91.5952601387199$$
$$x_{37} = 73.5677818817124$$
$$x_{38} = 37.8696025768297$$
$$x_{39} = -1.83044180444259$$
$$x_{40} = 57.9070167414894$$
$$x_{41} = 1.35834897687309$$
$$x_{42} = -8.06642898389629$$
$$x_{43} = -36.3407628662044$$
$$x_{44} = 25.6218144133343$$
$$x_{45} = -65.8029549916335$$
$$x_{46} = -64.6150967485126$$
$$x_{47} = 47.2943805375991$$
$$x_{48} = 66.1439364591379$$
$$x_{49} = -12.0772974297432$$
$$x_{50} = -30.926853351282$$
$$x_{51} = 86.1813506237975$$
$$x_{52} = 101.020038099489$$
$$x_{53} = -75.2277329524028$$
$$x_{54} = 20.2079048984118$$
$$x_{55} = 48.48223878072$$
$$x_{56} = 16.1970364525649$$
$$x_{57} = 76.7565726630281$$
$$x_{58} = -67.8038875298282$$
$$x_{59} = -78.0507431551289$$
$$x_{60} = 26.4438920778655$$
$$x_{61} = -68.6259651943595$$
$$x_{62} = 64.1430039209431$$
$$x_{63} = -59.2011872335901$$
$$x_{64} = -17.4912069446657$$
$$x_{65} = 72.7457042171812$$
$$x_{66} = 29.6326828591812$$
$$x_{67} = -40.3516313120514$$
$$x_{68} = -26.915984905435$$
$$x_{69} = 94.418270341446$$
$$x_{70} = -281.384989846208$$
$$x_{71} = 44.471370334873$$
The values of the extrema at the points:
(35.04659237410367, 1.4407155084997)
(-28.103843148555935, -0.0281049572158007)
(-87.47552111589827, 1.44071550849969)
(-96.07822141213639, 1.63322372751841)
(95.60612858456689, 0.469048995851696)
(-9.254287227017176, -0.0281049572158003)
(-944.3082378813806, 1.63322372751836)
(-94.0772888739416, -0.0281049572158013)
(53.896148295642426, 1.4407155084997)
(-2.6525194689738547, 1.4407155084997)
(35.868670038634924, 1.63322372751842)
(-37.528621109325314, -0.0281049572158007)
(-39.52955364752011, 1.63322372751841)
(-74.03987470928195, 0.469048995851693)
(-49.77640927282075, 1.4407155084997)
(92.4173378032512, 1.63322372751842)
(-45.76554082697381, 0.469048995851693)
(67.33179470225875, 0.469048995851694)
(-21.502075390512616, 1.4407155084997)
(56.71915849836848, -0.0281049572157989)
(-58.37910956905887, 1.63322372751841)
(-56.37817703086407, -0.0281049572158025)
(63.3209262564118, 1.4407155084997)
(-96.90029907666765, 1.44071550849968)
(7.594336156326788, 1.63322372751841)
(8180.877760681574, -0.028104957215496)
(10.783126937642468, 0.469048995851694)
(0.17049073375220267, -0.0281049572158001)
(82.17048217795056, 1.4407155084997)
(28.44482461606034, -0.0281049572157989)
(-30.10477568675073, 1.63322372751841)
(9.595268694521582, -0.0281049572157998)
(6.772258491795525, 1.4407155084997)
(-111.73898655235946, 0.469048995851688)
(-83.46465267005134, 0.469048995851693)
(91.59526013871994, 1.4407155084997)
(73.56778188171245, 1.63322372751841)
(37.86960257682972, -0.0281049572157989)
(-1.8304418044425919, 1.63322372751841)
(57.907016741489365, 0.469048995851697)
(1.3583489768730883, 0.469048995851694)
(-8.06642898389629, 0.469048995851694)
(-36.34076286620443, 0.469048995851693)
(25.621814413334285, 1.4407155084997)
(-65.80295499163346, -0.0281049572158001)
(-64.61509674851257, 0.469048995851691)
(47.2943805375991, -0.0281049572157995)
(66.14393645913786, -0.0281049572157978)
(-12.077297429743235, 1.44071550849969)
(-30.926853351281995, 1.44071550849969)
(86.1813506237975, 0.469048995851696)
(101.02003809948933, 1.4407155084997)
(-75.22773295240283, -0.0281049572158025)
(20.207904898411847, 0.469048995851695)
(48.48223878071999, 0.469048995851694)
(16.197036452564905, 1.4407155084997)
(76.75657266302812, 0.469048995851697)
(-67.80388752982824, 1.63322372751841)
(-78.0507431551289, 1.4407155084997)
(26.44389207786555, 1.63322372751842)
(-68.62596519435951, 1.4407155084997)
(64.14300392094307, 1.63322372751842)
(-59.20118723359013, 1.44071550849969)
(-17.49120694466567, 0.469048995851694)
(72.74570421718118, 1.4407155084997)
(29.632682859181227, 0.469048995851695)
(-40.351631312051374, 1.44071550849969)
(-26.91598490543505, 0.469048995851693)
(94.418270341446, -0.0281049572158001)
(-281.3849898462083, 0.469048995851682)
(44.47137033487304, 1.4407155084997)
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:
$$x_{1} = 35.0465923741037$$
$$x_{2} = -28.1038431485559$$
$$x_{3} = -87.4755211158983$$
$$x_{4} = -9.25428722701718$$
$$x_{5} = -94.0772888739416$$
$$x_{6} = 53.8961482956424$$
$$x_{7} = -2.65251946897385$$
$$x_{8} = -37.5286211093253$$
$$x_{9} = -49.7764092728208$$
$$x_{10} = -21.5020753905126$$
$$x_{11} = 56.7191584983685$$
$$x_{12} = -56.3781770308641$$
$$x_{13} = 63.3209262564118$$
$$x_{14} = -96.9002990766677$$
$$x_{15} = 8180.87776068157$$
$$x_{16} = 0.170490733752203$$
$$x_{17} = 82.1704821779506$$
$$x_{18} = 28.4448246160603$$
$$x_{19} = 9.59526869452158$$
$$x_{20} = 6.77225849179553$$
$$x_{21} = 91.5952601387199$$
$$x_{22} = 37.8696025768297$$
$$x_{23} = 25.6218144133343$$
$$x_{24} = -65.8029549916335$$
$$x_{25} = 47.2943805375991$$
$$x_{26} = 66.1439364591379$$
$$x_{27} = -12.0772974297432$$
$$x_{28} = -30.926853351282$$
$$x_{29} = 101.020038099489$$
$$x_{30} = -75.2277329524028$$
$$x_{31} = 16.1970364525649$$
$$x_{32} = -78.0507431551289$$
$$x_{33} = -68.6259651943595$$
$$x_{34} = -59.2011872335901$$
$$x_{35} = 72.7457042171812$$
$$x_{36} = -40.3516313120514$$
$$x_{37} = 94.418270341446$$
$$x_{38} = 44.471370334873$$
Maxima of the function at points:
$$x_{38} = -96.0782214121364$$
$$x_{38} = 95.6061285845669$$
$$x_{38} = -944.308237881381$$
$$x_{38} = 35.8686700386349$$
$$x_{38} = -39.5295536475201$$
$$x_{38} = -74.0398747092819$$
$$x_{38} = 92.4173378032512$$
$$x_{38} = -45.7655408269738$$
$$x_{38} = 67.3317947022587$$
$$x_{38} = -58.3791095690589$$
$$x_{38} = 7.59433615632679$$
$$x_{38} = 10.7831269376425$$
$$x_{38} = -30.1047756867507$$
$$x_{38} = -111.738986552359$$
$$x_{38} = -83.4646526700513$$
$$x_{38} = 73.5677818817124$$
$$x_{38} = -1.83044180444259$$
$$x_{38} = 57.9070167414894$$
$$x_{38} = 1.35834897687309$$
$$x_{38} = -8.06642898389629$$
$$x_{38} = -36.3407628662044$$
$$x_{38} = -64.6150967485126$$
$$x_{38} = 86.1813506237975$$
$$x_{38} = 20.2079048984118$$
$$x_{38} = 48.48223878072$$
$$x_{38} = 76.7565726630281$$
$$x_{38} = -67.8038875298282$$
$$x_{38} = 26.4438920778655$$
$$x_{38} = 64.1430039209431$$
$$x_{38} = -17.4912069446657$$
$$x_{38} = 29.6326828591812$$
$$x_{38} = -26.915984905435$$
$$x_{38} = -281.384989846208$$
Decreasing at intervals
$$\left[8180.87776068157, \infty\right)$$
Increasing at intervals
$$\left(-\infty, -96.9002990766677\right]$$