In order to find the extrema, we need to solve the equation
$$\frac{d}{d y} f{\left(y \right)} = 0$$
(the derivative equals zero),
and the roots of this equation are the extrema of this function:
$$\frac{d}{d y} f{\left(y \right)} = $$
the first derivative$$\frac{\cos{\left(y \right)}}{y^{3}} - \frac{3 \sin{\left(y \right)}}{y^{4}} = 0$$
Solve this equationThe roots of this equation
$$y_{1} = 7.47219265966058$$
$$y_{2} = -48.6330777853047$$
$$y_{3} = 48.6330777853047$$
$$y_{4} = 86.359073259985$$
$$y_{5} = -13.924969952549$$
$$y_{6} = 64.3560674689022$$
$$y_{7} = -80.0731644462726$$
$$y_{8} = -51.7784042739041$$
$$y_{9} = 73.7867920572034$$
$$y_{10} = 95.7872667660245$$
$$y_{11} = 45.4872362867621$$
$$y_{12} = -108.357267428671$$
$$y_{13} = -26.591193287969$$
$$y_{14} = -23.4346216921802$$
$$y_{15} = -20.2734415170608$$
$$y_{16} = 76.930043294192$$
$$y_{17} = 32.8957773192946$$
$$y_{18} = 39.1935138550425$$
$$y_{19} = -95.7872667660245$$
$$y_{20} = -76.930043294192$$
$$y_{21} = -42.3407653325706$$
$$y_{22} = -73.7867920572034$$
$$y_{23} = 80.0731644462726$$
$$y_{24} = -61.2120859995403$$
$$y_{25} = 83.2161702400252$$
$$y_{26} = -83.2161702400252$$
$$y_{27} = 98.9298533613919$$
$$y_{28} = 67.4998267230665$$
$$y_{29} = -98.9298533613919$$
$$y_{30} = -186.908713650658$$
$$y_{31} = 10.7227710626892$$
$$y_{32} = 89.5018843244389$$
$$y_{33} = 17.1051395364267$$
$$y_{34} = 36.0452782424582$$
$$y_{35} = -86.359073259985$$
$$y_{36} = -29.7446115259422$$
$$y_{37} = 23.4346216921802$$
$$y_{38} = 92.6446127847888$$
$$y_{39} = 4.07814976485137$$
$$y_{40} = 54.9233040395155$$
$$y_{41} = 61.2120859995403$$
$$y_{42} = -7.47219265966058$$
$$y_{43} = 29.7446115259422$$
$$y_{44} = -39.1935138550425$$
$$y_{45} = -58.0678462801751$$
$$y_{46} = -70.6433933906731$$
$$y_{47} = 3215.41914794509$$
$$y_{48} = 58.0678462801751$$
$$y_{49} = 20.2734415170608$$
$$y_{50} = -67.4998267230665$$
$$y_{51} = 42.3407653325706$$
$$y_{52} = 26.591193287969$$
$$y_{53} = -17.1051395364267$$
$$y_{54} = -54.9233040395155$$
$$y_{55} = 70.6433933906731$$
$$y_{56} = -32.8957773192946$$
$$y_{57} = -4.07814976485137$$
$$y_{58} = 13.924969952549$$
$$y_{59} = -64.3560674689022$$
$$y_{60} = -89.5018843244389$$
$$y_{61} = 51.7784042739041$$
$$y_{62} = -45.4872362867621$$
$$y_{63} = -92.6446127847888$$
$$y_{64} = -10.7227710626892$$
$$y_{65} = -36.0452782424582$$
The values of the extrema at the points:
(7.472192659660579, 0.00222435242575847)
(-48.63307778530466, -8.67720806398467e-6)
(48.63307778530466, -8.67720806398467e-6)
(86.359073259985, -1.55172297524868e-6)
(-13.92496995254897, 0.000362047304723488)
(64.35606746890217, 3.7476597286644e-6)
(-80.07316444627257, -1.94641047170913e-6)
(-51.77840427390411, 7.1916125507828e-6)
(73.78679205720341, -2.48716990126569e-6)
(95.78726676602449, 1.1372704641271e-6)
(45.48723628676209, 1.06020259212848e-5)
(-108.35726742867121, 7.85704936475449e-7)
(-26.59119328796898, 5.28494009082656e-5)
(-23.4346216921802, -7.70719574291125e-5)
(-20.27344151706078, 0.000118717289027919)
(76.93004329419203, 2.19473504875366e-6)
(32.89577731929462, 2.79757033726946e-5)
(39.193513855042454, 1.65610881918558e-5)
(-95.78726676602449, 1.1372704641271e-6)
(-76.93004329419203, 2.19473504875366e-6)
(-42.34076533257061, -1.31412441116291e-5)
(-73.78679205720341, -2.48716990126569e-6)
(80.07316444627257, -1.94641047170913e-6)
(-61.21208599954033, -4.35479288166118e-6)
(83.21617024002518, 1.73418247352317e-6)
(-83.21617024002518, 1.73418247352317e-6)
(98.9298533613919, -1.03232943885669e-6)
(67.49982672306646, -3.24835521431348e-6)
(-98.9298533613919, -1.03232943885669e-6)
(-186.90871365065752, -1.53128285331065e-7)
(10.722771062689203, -0.00078111312666525)
(89.50188432443886, 1.39398991793862e-6)
(17.105139536426744, -0.000196807350078387)
(36.04527824245817, -2.12792275158681e-5)
(-86.359073259985, -1.55172297524868e-6)
(-29.744611525942226, -3.78074454700618e-5)
(23.4346216921802, -7.70719574291125e-5)
(92.64461278478876, -1.25693237122389e-6)
(4.078149764851372, -0.0118764951343876)
(54.92330403951548, -6.02674942320184e-6)
(61.21208599954033, -4.35479288166118e-6)
(-7.472192659660579, 0.00222435242575847)
(29.744611525942226, -3.78074454700618e-5)
(-39.193513855042454, 1.65610881918558e-5)
(-58.067846280175104, 5.1005149012716e-6)
(-70.64339339067311, 2.83396240646379e-6)
(3215.41914794509, -3.00806370783767e-11)
(58.067846280175104, 5.1005149012716e-6)
(20.27344151706078, 0.000118717289027919)
(-67.49982672306646, -3.24835521431348e-6)
(42.34076533257061, -1.31412441116291e-5)
(26.59119328796898, 5.28494009082656e-5)
(-17.105139536426744, -0.000196807350078387)
(-54.92330403951548, -6.02674942320184e-6)
(70.64339339067311, 2.83396240646379e-6)
(-32.89577731929462, 2.79757033726946e-5)
(-4.078149764851372, -0.0118764951343876)
(13.92496995254897, 0.000362047304723488)
(-64.35606746890217, 3.7476597286644e-6)
(-89.50188432443886, 1.39398991793862e-6)
(51.77840427390411, 7.1916125507828e-6)
(-45.48723628676209, 1.06020259212848e-5)
(-92.64461278478876, -1.25693237122389e-6)
(-10.722771062689203, -0.00078111312666525)
(-36.04527824245817, -2.12792275158681e-5)
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:
$$y_{1} = -48.6330777853047$$
$$y_{2} = 48.6330777853047$$
$$y_{3} = 86.359073259985$$
$$y_{4} = -80.0731644462726$$
$$y_{5} = 73.7867920572034$$
$$y_{6} = -23.4346216921802$$
$$y_{7} = -42.3407653325706$$
$$y_{8} = -73.7867920572034$$
$$y_{9} = 80.0731644462726$$
$$y_{10} = -61.2120859995403$$
$$y_{11} = 98.9298533613919$$
$$y_{12} = 67.4998267230665$$
$$y_{13} = -98.9298533613919$$
$$y_{14} = -186.908713650658$$
$$y_{15} = 10.7227710626892$$
$$y_{16} = 17.1051395364267$$
$$y_{17} = 36.0452782424582$$
$$y_{18} = -86.359073259985$$
$$y_{19} = -29.7446115259422$$
$$y_{20} = 23.4346216921802$$
$$y_{21} = 92.6446127847888$$
$$y_{22} = 4.07814976485137$$
$$y_{23} = 54.9233040395155$$
$$y_{24} = 61.2120859995403$$
$$y_{25} = 29.7446115259422$$
$$y_{26} = 3215.41914794509$$
$$y_{27} = -67.4998267230665$$
$$y_{28} = 42.3407653325706$$
$$y_{29} = -17.1051395364267$$
$$y_{30} = -54.9233040395155$$
$$y_{31} = -4.07814976485137$$
$$y_{32} = -92.6446127847888$$
$$y_{33} = -10.7227710626892$$
$$y_{34} = -36.0452782424582$$
Maxima of the function at points:
$$y_{34} = 7.47219265966058$$
$$y_{34} = -13.924969952549$$
$$y_{34} = 64.3560674689022$$
$$y_{34} = -51.7784042739041$$
$$y_{34} = 95.7872667660245$$
$$y_{34} = 45.4872362867621$$
$$y_{34} = -108.357267428671$$
$$y_{34} = -26.591193287969$$
$$y_{34} = -20.2734415170608$$
$$y_{34} = 76.930043294192$$
$$y_{34} = 32.8957773192946$$
$$y_{34} = 39.1935138550425$$
$$y_{34} = -95.7872667660245$$
$$y_{34} = -76.930043294192$$
$$y_{34} = 83.2161702400252$$
$$y_{34} = -83.2161702400252$$
$$y_{34} = 89.5018843244389$$
$$y_{34} = -7.47219265966058$$
$$y_{34} = -39.1935138550425$$
$$y_{34} = -58.0678462801751$$
$$y_{34} = -70.6433933906731$$
$$y_{34} = 58.0678462801751$$
$$y_{34} = 20.2734415170608$$
$$y_{34} = 26.591193287969$$
$$y_{34} = 70.6433933906731$$
$$y_{34} = -32.8957773192946$$
$$y_{34} = 13.924969952549$$
$$y_{34} = -64.3560674689022$$
$$y_{34} = -89.5018843244389$$
$$y_{34} = 51.7784042739041$$
$$y_{34} = -45.4872362867621$$
Decreasing at intervals
$$\left[3215.41914794509, \infty\right)$$
Increasing at intervals
$$\left(-\infty, -186.908713650658\right]$$