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$$- x \cos{\left(x \right)} - \sin{\left(x \right)} = 0$$
Solve this equationThe roots of this equation
$$x_{1} = -80.1230928148503$$
$$x_{2} = 92.687771772017$$
$$x_{3} = -70.69997803861$$
$$x_{4} = 54.9960525574964$$
$$x_{5} = -20.469167402741$$
$$x_{6} = -42.4350618814099$$
$$x_{7} = 0$$
$$x_{8} = -17.3363779239834$$
$$x_{9} = -45.57503179559$$
$$x_{10} = -76.9820093304187$$
$$x_{11} = 29.8785865061074$$
$$x_{12} = 45.57503179559$$
$$x_{13} = 89.5465575382492$$
$$x_{14} = 36.1559664195367$$
$$x_{15} = -29.8785865061074$$
$$x_{16} = -48.7152107175577$$
$$x_{17} = -23.6042847729804$$
$$x_{18} = 95.8290108090195$$
$$x_{19} = -58.1366632448992$$
$$x_{20} = -61.2773745335697$$
$$x_{21} = -4.91318043943488$$
$$x_{22} = -33.0170010333572$$
$$x_{23} = 76.9820093304187$$
$$x_{24} = -14.2074367251912$$
$$x_{25} = 42.4350618814099$$
$$x_{26} = 23.6042847729804$$
$$x_{27} = 14.2074367251912$$
$$x_{28} = -86.4053708116885$$
$$x_{29} = 70.69997803861$$
$$x_{30} = -39.295350981473$$
$$x_{31} = 20.469167402741$$
$$x_{32} = -26.7409160147873$$
$$x_{33} = 83.2642147040886$$
$$x_{34} = -64.4181717218392$$
$$x_{35} = 80.1230928148503$$
$$x_{36} = -89.5465575382492$$
$$x_{37} = -7.97866571241324$$
$$x_{38} = 33.0170010333572$$
$$x_{39} = 73.8409691490209$$
$$x_{40} = -83.2642147040886$$
$$x_{41} = 11.085538406497$$
$$x_{42} = 86.4053708116885$$
$$x_{43} = 51.855560729152$$
$$x_{44} = -51.855560729152$$
$$x_{45} = 4.91318043943488$$
$$x_{46} = -2.02875783811043$$
$$x_{47} = 26.7409160147873$$
$$x_{48} = 48.7152107175577$$
$$x_{49} = 39.295350981473$$
$$x_{50} = 67.5590428388084$$
$$x_{51} = -54.9960525574964$$
$$x_{52} = 64.4181717218392$$
$$x_{53} = 2.02875783811043$$
$$x_{54} = 98.9702722883957$$
$$x_{55} = -73.8409691490209$$
$$x_{56} = 61.2773745335697$$
$$x_{57} = 7.97866571241324$$
$$x_{58} = -67.5590428388084$$
$$x_{59} = 58.1366632448992$$
$$x_{60} = -92.687771772017$$
$$x_{61} = -98.9702722883957$$
$$x_{62} = 102.111554139654$$
$$x_{63} = -11.085538406497$$
$$x_{64} = -36.1559664195367$$
$$x_{65} = -95.8290108090195$$
$$x_{66} = 17.3363779239834$$
The values of the extrema at the points:
(-80.12309281485025, 80.1168531456592)
(92.687771772017, 92.6823777880592)
(-70.69997803861, -70.6929069615931)
(54.99605255749639, 54.9869632496976)
(-20.46916740274095, -20.4447840582523)
(-42.43506188140989, 42.4232840772591)
(0, 0)
(-17.33637792398336, 17.3076086078585)
(-45.57503179559002, -45.5640648360268)
(-76.98200933041872, -76.9755151282637)
(29.878586506107393, 29.8618661591868)
(45.57503179559002, -45.5640648360268)
(89.54655753824919, -89.5409743728852)
(36.15596641953672, 36.1421453722421)
(-29.878586506107393, 29.8618661591868)
(-48.715210717557724, 48.7049502253679)
(-23.604284772980407, 23.5831306496334)
(95.82901080901948, -95.8237936084657)
(-58.13666324489916, -58.1280647280857)
(-61.277374533569656, 61.2692165444766)
(-4.913180439434884, 4.81446988971227)
(-33.017001033357246, -33.0018677308454)
(76.98200933041872, -76.9755151282637)
(-14.207436725191188, -14.1723741137743)
(42.43506188140989, 42.4232840772591)
(23.604284772980407, 23.5831306496334)
(14.207436725191188, -14.1723741137743)
(-86.40537081168854, 86.3995847156108)
(70.69997803861, -70.6929069615931)
(-39.295350981472986, -39.2826330068918)
(20.46916740274095, -20.4447840582523)
(-26.74091601478731, -26.7222376646974)
(83.26421470408864, -83.2582103729533)
(-64.41817172183916, -64.4104113393753)
(80.12309281485025, 80.1168531456592)
(-89.54655753824919, -89.5409743728852)
(-7.978665712413241, -7.91672737158778)
(33.017001033357246, -33.0018677308454)
(73.8409691490209, 73.8341987715416)
(-83.26421470408864, -83.2582103729533)
(11.085538406497022, 11.04070801593)
(86.40537081168854, 86.3995847156108)
(51.85556072915197, -51.8459212502015)
(-51.85556072915197, -51.8459212502015)
(4.913180439434884, 4.81446988971227)
(-2.028757838110434, -1.81970574115965)
(26.74091601478731, -26.7222376646974)
(48.715210717557724, 48.7049502253679)
(39.295350981472986, -39.2826330068918)
(67.5590428388084, 67.5516431209725)
(-54.99605255749639, 54.9869632496976)
(64.41817172183916, -64.4104113393753)
(2.028757838110434, -1.81970574115965)
(98.9702722883957, 98.9652206531187)
(-73.8409691490209, 73.8341987715416)
(61.277374533569656, 61.2692165444766)
(7.978665712413241, -7.91672737158778)
(-67.5590428388084, 67.5516431209725)
(58.13666324489916, -58.1280647280857)
(-92.687771772017, 92.6823777880592)
(-98.9702722883957, 98.9652206531187)
(102.11155413965392, -102.106657886316)
(-11.085538406497022, 11.04070801593)
(-36.15596641953672, 36.1421453722421)
(-95.82901080901948, -95.8237936084657)
(17.33637792398336, 17.3076086078585)
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:
$$x_{1} = -70.69997803861$$
$$x_{2} = -20.469167402741$$
$$x_{3} = -45.57503179559$$
$$x_{4} = -76.9820093304187$$
$$x_{5} = 45.57503179559$$
$$x_{6} = 89.5465575382492$$
$$x_{7} = 95.8290108090195$$
$$x_{8} = -58.1366632448992$$
$$x_{9} = -33.0170010333572$$
$$x_{10} = 76.9820093304187$$
$$x_{11} = -14.2074367251912$$
$$x_{12} = 14.2074367251912$$
$$x_{13} = 70.69997803861$$
$$x_{14} = -39.295350981473$$
$$x_{15} = 20.469167402741$$
$$x_{16} = -26.7409160147873$$
$$x_{17} = 83.2642147040886$$
$$x_{18} = -64.4181717218392$$
$$x_{19} = -89.5465575382492$$
$$x_{20} = -7.97866571241324$$
$$x_{21} = 33.0170010333572$$
$$x_{22} = -83.2642147040886$$
$$x_{23} = 51.855560729152$$
$$x_{24} = -51.855560729152$$
$$x_{25} = -2.02875783811043$$
$$x_{26} = 26.7409160147873$$
$$x_{27} = 39.295350981473$$
$$x_{28} = 64.4181717218392$$
$$x_{29} = 2.02875783811043$$
$$x_{30} = 7.97866571241324$$
$$x_{31} = 58.1366632448992$$
$$x_{32} = 102.111554139654$$
$$x_{33} = -95.8290108090195$$
Maxima of the function at points:
$$x_{33} = -80.1230928148503$$
$$x_{33} = 92.687771772017$$
$$x_{33} = 54.9960525574964$$
$$x_{33} = -42.4350618814099$$
$$x_{33} = 0$$
$$x_{33} = -17.3363779239834$$
$$x_{33} = 29.8785865061074$$
$$x_{33} = 36.1559664195367$$
$$x_{33} = -29.8785865061074$$
$$x_{33} = -48.7152107175577$$
$$x_{33} = -23.6042847729804$$
$$x_{33} = -61.2773745335697$$
$$x_{33} = -4.91318043943488$$
$$x_{33} = 42.4350618814099$$
$$x_{33} = 23.6042847729804$$
$$x_{33} = -86.4053708116885$$
$$x_{33} = 80.1230928148503$$
$$x_{33} = 73.8409691490209$$
$$x_{33} = 11.085538406497$$
$$x_{33} = 86.4053708116885$$
$$x_{33} = 4.91318043943488$$
$$x_{33} = 48.7152107175577$$
$$x_{33} = 67.5590428388084$$
$$x_{33} = -54.9960525574964$$
$$x_{33} = 98.9702722883957$$
$$x_{33} = -73.8409691490209$$
$$x_{33} = 61.2773745335697$$
$$x_{33} = -67.5590428388084$$
$$x_{33} = -92.687771772017$$
$$x_{33} = -98.9702722883957$$
$$x_{33} = -11.085538406497$$
$$x_{33} = -36.1559664195367$$
$$x_{33} = 17.3363779239834$$
Decreasing at intervals
$$\left[102.111554139654, \infty\right)$$
Increasing at intervals
$$\left(-\infty, -95.8290108090195\right]$$