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$$\frac{\cos{\left(x \right)}}{x^{4}} - \frac{4 \sin{\left(x \right)}}{x^{5}} = 0$$
Solve this equationThe roots of this equation
$$x_{1} = 58.0506675115744$$
$$x_{2} = -26.554025372648$$
$$x_{3} = -67.4850389217693$$
$$x_{4} = -86.3475066265478$$
$$x_{5} = 102.062589651767$$
$$x_{6} = -3.91643536817833$$
$$x_{7} = 61.1957856169513$$
$$x_{8} = -95.7768364391699$$
$$x_{9} = -58.0506675115744$$
$$x_{10} = -83.2041677899554$$
$$x_{11} = -23.3925885027245$$
$$x_{12} = 39.1681371651634$$
$$x_{13} = 7.35592702313142$$
$$x_{14} = 73.7732602110395$$
$$x_{15} = 45.4653403180975$$
$$x_{16} = -48.6125878655427$$
$$x_{17} = -80.0606920801604$$
$$x_{18} = 42.3172567562625$$
$$x_{19} = -17.0483006782016$$
$$x_{20} = -98.9197537891963$$
$$x_{21} = 76.9170627514565$$
$$x_{22} = 20.2250979158179$$
$$x_{23} = -73.7732602110395$$
$$x_{24} = -70.6292613872399$$
$$x_{25} = 95.7768364391699$$
$$x_{26} = 98.9197537891963$$
$$x_{27} = 120.918249020695$$
$$x_{28} = 51.7591510649689$$
$$x_{29} = 70.6292613872399$$
$$x_{30} = 64.3405601260236$$
$$x_{31} = -45.4653403180975$$
$$x_{32} = 89.4907229874848$$
$$x_{33} = -13.856126658747$$
$$x_{34} = 26.554025372648$$
$$x_{35} = 54.905147004153$$
$$x_{36} = -39.1681371651634$$
$$x_{37} = 13.856126658747$$
$$x_{38} = -10.6358514209244$$
$$x_{39} = 67.4850389217693$$
$$x_{40} = 252.882392303412$$
$$x_{41} = 80.0606920801604$$
$$x_{42} = -89.4907229874848$$
$$x_{43} = 86.3475066265478$$
$$x_{44} = -76.9170627514565$$
$$x_{45} = -32.8656107569429$$
$$x_{46} = -61.1957856169513$$
$$x_{47} = -36.0177122696989$$
$$x_{48} = -64.3405601260236$$
$$x_{49} = -20.2250979158179$$
$$x_{50} = -142.914484277841$$
$$x_{51} = 10.6358514209244$$
$$x_{52} = -922.053105710894$$
$$x_{53} = 48.6125878655427$$
$$x_{54} = 29.7113059713865$$
$$x_{55} = 23.3925885027245$$
$$x_{56} = -92.6338293197393$$
$$x_{57} = -7.35592702313142$$
$$x_{58} = 92.6338293197393$$
$$x_{59} = -42.3172567562625$$
$$x_{60} = 17.0483006782016$$
$$x_{61} = -51.7591510649689$$
$$x_{62} = 32.8656107569429$$
$$x_{63} = 83.2041677899554$$
$$x_{64} = -54.905147004153$$
$$x_{65} = 36.0177122696989$$
$$x_{66} = -29.7113059713865$$
$$x_{67} = 3.91643536817833$$
The values of the extrema at the points:
(58.05066751157438, 8.78501616937435e-8)
(-26.554025372648002, -1.98886935969342e-6)
(-67.48503892176933, 4.81291775980568e-8)
(-86.3475066265478, 1.7969471563081e-8)
(102.06258965176721, 9.20874372329637e-9)
(-3.916435368178333, 0.00297363898119923)
(61.195785616951255, -7.11521729092172e-8)
(-95.77683643916993, -1.1873523810379e-8)
(-58.05066751157438, -8.78501616937435e-8)
(-83.20416778995545, -2.08409922801071e-8)
(-23.392588502724486, 3.29176440749978e-6)
(39.168137165163394, 4.22683574600292e-7)
(7.355927023131424, 0.000300053589186415)
(73.77326021103951, -3.37106134268981e-8)
(45.46534031809747, 2.33133077593193e-7)
(-48.612587865542714, 1.78459548054671e-7)
(-80.06069208016042, 2.43097935138504e-8)
(42.31725675626252, -3.10454831057143e-7)
(-17.0483006782016, 1.15249508377501e-5)
(-98.91975378919633, 1.04354965234194e-8)
(76.91706275145651, 2.85313794171889e-8)
(20.22509791581794, 5.86280903576885e-6)
(-73.77326021103951, 3.37106134268981e-8)
(-70.62926138723986, -4.01204672332662e-8)
(95.77683643916993, 1.1873523810379e-8)
(98.91975378919633, -1.04354965234194e-8)
(120.91824902069463, 4.67514523669302e-9)
(51.759151064968904, 1.38917960418904e-7)
(70.62926138723986, 4.01204672332662e-8)
(64.34056012602356, 5.82402180776872e-8)
(-45.46534031809747, -2.33133077593193e-7)
(89.49072298748483, 1.55759497424426e-8)
(-13.856126658747035, -2.60645897698916e-5)
(26.554025372648002, 1.98886935969342e-6)
(54.90514700415301, -1.09748424351353e-7)
(-39.168137165163394, -4.22683574600292e-7)
(13.856126658747035, 2.60645897698916e-5)
(-10.635851420924443, 7.31449268885113e-5)
(67.48503892176933, -4.81291775980568e-8)
(252.88239230341162, 2.44495748568497e-10)
(80.06069208016042, -2.43097935138504e-8)
(-89.49072298748483, -1.55759497424426e-8)
(86.3475066265478, -1.7969471563081e-8)
(-76.91706275145651, -2.85313794171889e-8)
(-32.86561075694293, -8.50824929781849e-7)
(-61.195785616951255, 7.11521729092172e-8)
(-36.017712269698876, 5.90573138490323e-7)
(-64.34056012602356, -5.82402180776872e-8)
(-20.22509791581794, -5.86280903576885e-6)
(-142.9144842778405, 2.39621053902668e-9)
(10.635851420924443, -7.31449268885113e-5)
(-922.0531057108941, 1.38347773259507e-12)
(48.612587865542714, -1.78459548054671e-7)
(29.711305971386484, -1.27178147734334e-6)
(23.392588502724486, -3.29176440749978e-6)
(-92.63382931973935, 1.35680370015572e-8)
(-7.355927023131424, -0.000300053589186415)
(92.63382931973935, -1.35680370015572e-8)
(-42.31725675626252, 3.10454831057143e-7)
(17.0483006782016, -1.15249508377501e-5)
(-51.759151064968904, -1.38917960418904e-7)
(32.86561075694293, 8.50824929781849e-7)
(83.20416778995545, 2.08409922801071e-8)
(-54.90514700415301, 1.09748424351353e-7)
(36.017712269698876, -5.90573138490323e-7)
(-29.711305971386484, 1.27178147734334e-6)
(3.916435368178333, -0.00297363898119923)
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:
$$x_{1} = -26.554025372648$$
$$x_{2} = 61.1957856169513$$
$$x_{3} = -95.7768364391699$$
$$x_{4} = -58.0506675115744$$
$$x_{5} = -83.2041677899554$$
$$x_{6} = 73.7732602110395$$
$$x_{7} = 42.3172567562625$$
$$x_{8} = -70.6292613872399$$
$$x_{9} = 98.9197537891963$$
$$x_{10} = -45.4653403180975$$
$$x_{11} = -13.856126658747$$
$$x_{12} = 54.905147004153$$
$$x_{13} = -39.1681371651634$$
$$x_{14} = 67.4850389217693$$
$$x_{15} = 80.0606920801604$$
$$x_{16} = -89.4907229874848$$
$$x_{17} = 86.3475066265478$$
$$x_{18} = -76.9170627514565$$
$$x_{19} = -32.8656107569429$$
$$x_{20} = -64.3405601260236$$
$$x_{21} = -20.2250979158179$$
$$x_{22} = 10.6358514209244$$
$$x_{23} = 48.6125878655427$$
$$x_{24} = 29.7113059713865$$
$$x_{25} = 23.3925885027245$$
$$x_{26} = -7.35592702313142$$
$$x_{27} = 92.6338293197393$$
$$x_{28} = 17.0483006782016$$
$$x_{29} = -51.7591510649689$$
$$x_{30} = 36.0177122696989$$
$$x_{31} = 3.91643536817833$$
Maxima of the function at points:
$$x_{31} = 58.0506675115744$$
$$x_{31} = -67.4850389217693$$
$$x_{31} = -86.3475066265478$$
$$x_{31} = 102.062589651767$$
$$x_{31} = -3.91643536817833$$
$$x_{31} = -23.3925885027245$$
$$x_{31} = 39.1681371651634$$
$$x_{31} = 7.35592702313142$$
$$x_{31} = 45.4653403180975$$
$$x_{31} = -48.6125878655427$$
$$x_{31} = -80.0606920801604$$
$$x_{31} = -17.0483006782016$$
$$x_{31} = -98.9197537891963$$
$$x_{31} = 76.9170627514565$$
$$x_{31} = 20.2250979158179$$
$$x_{31} = -73.7732602110395$$
$$x_{31} = 95.7768364391699$$
$$x_{31} = 120.918249020695$$
$$x_{31} = 51.7591510649689$$
$$x_{31} = 70.6292613872399$$
$$x_{31} = 64.3405601260236$$
$$x_{31} = 89.4907229874848$$
$$x_{31} = 26.554025372648$$
$$x_{31} = 13.856126658747$$
$$x_{31} = -10.6358514209244$$
$$x_{31} = 252.882392303412$$
$$x_{31} = -61.1957856169513$$
$$x_{31} = -36.0177122696989$$
$$x_{31} = -142.914484277841$$
$$x_{31} = -922.053105710894$$
$$x_{31} = -92.6338293197393$$
$$x_{31} = -42.3172567562625$$
$$x_{31} = 32.8656107569429$$
$$x_{31} = 83.2041677899554$$
$$x_{31} = -54.905147004153$$
$$x_{31} = -29.7113059713865$$
Decreasing at intervals
$$\left[98.9197537891963, \infty\right)$$
Increasing at intervals
$$\left(-\infty, -95.7768364391699\right]$$