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$$2 \frac{1}{5 x} \cos{\left(2 x \right)} - \frac{\sin{\left(2 x \right)}}{5 x^{2}} = 0$$
Solve this equationThe roots of this equation
$$x_{1} = -3.86262591846885$$
$$x_{2} = 22.7655670069956$$
$$x_{3} = 60.4715244985757$$
$$x_{4} = 98.172223901556$$
$$x_{5} = 74.6094747920599$$
$$x_{6} = 2.24670472895453$$
$$x_{7} = -41.6200962353617$$
$$x_{8} = -60.4715244985757$$
$$x_{9} = 13.3330271294063$$
$$x_{10} = -93.4597065202651$$
$$x_{11} = 76.1803402100956$$
$$x_{12} = 18.0503111221878$$
$$x_{13} = 54.1878598258373$$
$$x_{14} = 82.4637755597094$$
$$x_{15} = 77.7512028363303$$
$$x_{16} = -19.6222161805821$$
$$x_{17} = -32.1935597952787$$
$$x_{18} = -57.3297052975115$$
$$x_{19} = -2.24670472895453$$
$$x_{20} = -24.3370721159772$$
$$x_{21} = -49.4750314121659$$
$$x_{22} = -98.172223901556$$
$$x_{23} = -99.7430603324317$$
$$x_{24} = 33.7647173885721$$
$$x_{25} = -46.3330961388114$$
$$x_{26} = -85.6054794697228$$
$$x_{27} = -69.8968599047927$$
$$x_{28} = 204.987701063789$$
$$x_{29} = 68.3259813506395$$
$$x_{30} = 90.3180208221014$$
$$x_{31} = -27.4798391439445$$
$$x_{32} = -652.665490738742$$
$$x_{33} = 30.6223651301872$$
$$x_{34} = -84.0346285545694$$
$$x_{35} = 32.1935597952787$$
$$x_{36} = -40.0490643144726$$
$$x_{37} = -10.1856514796438$$
$$x_{38} = -33.7647173885721$$
$$x_{39} = 41.6200962353617$$
$$x_{40} = -5.45206082971445$$
$$x_{41} = 40.0490643144726$$
$$x_{42} = 3.86262591846885$$
$$x_{43} = 11.7597262493445$$
$$x_{44} = 85.6054794697228$$
$$x_{45} = 52.6169257678188$$
$$x_{46} = -38.4780131551656$$
$$x_{47} = -25.9084912436398$$
$$x_{48} = -90.3180208221014$$
$$x_{49} = -71.4677348441946$$
$$x_{50} = 46.3330961388114$$
$$x_{51} = -13.3330271294063$$
$$x_{52} = -16.4781945199112$$
$$x_{53} = 19.6222161805821$$
$$x_{54} = 91.8888644664832$$
$$x_{55} = -35.3358428558098$$
$$x_{56} = 25.9084912436398$$
$$x_{57} = -58.9006179191122$$
$$x_{58} = -68.3259813506395$$
$$x_{59} = -76.1803402100956$$
$$x_{60} = -55.7587861230655$$
$$x_{61} = 10.1856514796438$$
$$x_{62} = 8.61037763596538$$
$$x_{63} = 66.7550989265392$$
$$x_{64} = -77.7512028363303$$
$$x_{65} = 63.6133213216672$$
$$x_{66} = 62.0424254948814$$
$$x_{67} = 55.7587861230655$$
$$x_{68} = -79.3220628366317$$
$$x_{69} = -63.6133213216672$$
$$x_{70} = -18.0503111221878$$
$$x_{71} = 16.4781945199112$$
$$x_{72} = 24.3370721159772$$
$$x_{73} = -62.0424254948814$$
$$x_{74} = 96.6013861664138$$
$$x_{75} = 99.7430603324317$$
$$x_{76} = 88.7471755026564$$
$$x_{77} = 51.0459832324538$$
$$x_{78} = -11.7597262493445$$
$$x_{79} = 38.4780131551656$$
$$x_{80} = -54.1878598258373$$
$$x_{81} = -82.4637755597094$$
$$x_{82} = 69.8968599047927$$
$$x_{83} = 84.0346285545694$$
$$x_{84} = 44.7621104652086$$
$$x_{85} = -91.8888644664832$$
$$x_{86} = 47.9040693934309$$
$$x_{87} = -47.9040693934309$$
The values of the extrema at the points:
(-3.8626259184688534, 0.0513498214103597)
(22.76556700699564, 0.00878307929139297)
(60.47152449857575, 0.00330722874014303)
(98.172223901556, 0.0020372096927757)
(74.60947479205991, -0.00268056449419756)
(2.246704728954532, -0.0868934512844887)
(-41.6200962353617, 0.00480502419282109)
(-60.47152449857575, 0.00330722874014303)
(13.333027129406338, 0.0149898079975725)
(-93.45970652026512, -0.00213992901717524)
(76.18034021009562, 0.00262529271726657)
(18.050311122187804, -0.0110758929204597)
(54.18785982583734, 0.00369070650031279)
(82.46377555970939, 0.0024252627583641)
(77.75120283633034, -0.00257225428487172)
(-19.622216180582097, 0.0101892212371523)
(-32.19355979527871, 0.00621167352298453)
(-57.32970529751154, 0.00348846017908172)
(-2.246704728954532, -0.0868934512844887)
(-24.337072115977193, -0.00821618161670149)
(-49.47503141216594, -0.00404223669460161)
(-98.172223901556, 0.0020372096927757)
(-99.74306033243167, -0.00200512683773813)
(33.76471738857206, -0.00592269357861969)
(-46.33309613881142, -0.0043163175398137)
(-85.60547946972281, 0.00233625919617385)
(-69.8968599047927, 0.00286128566232707)
(204.98770106378876, 0.000975665388749515)
(68.3259813506395, -0.00292706582698748)
(90.31802082210145, -0.00221436357359601)
(-27.479839143944467, -0.00727685852872126)
(-652.6654907387419, -0.000306435600087234)
(30.6223651301872, -0.00653030372839913)
(-84.0346285545694, -0.00237992912409497)
(32.19355979527871, 0.00621167352298453)
(-40.04906431447256, -0.0049934853287116)
(-10.18565147964378, 0.0196118496056297)
(-33.76471738857206, -0.00592269357861969)
(41.6200962353617, 0.00480502419282109)
(-5.4520608297144495, -0.0365300811292231)
(40.04906431447256, -0.0049934853287116)
(3.8626259184688534, 0.0513498214103597)
(11.759726249344503, -0.0169918467910451)
(85.60547946972281, 0.00233625919617385)
(52.6169257678188, -0.00380088664751342)
(-38.47801315516559, 0.00519733479481709)
(-25.908491243639833, 0.00771803979503518)
(-90.31802082210145, -0.00221436357359601)
(-71.46773484419464, -0.00279839715060906)
(46.33309613881142, -0.0043163175398137)
(-13.333027129406338, 0.0149898079975725)
(-16.478194519911238, 0.0121316684745241)
(19.622216180582097, 0.0101892212371523)
(91.88886446648316, 0.00217651007433517)
(-35.33584285580975, 0.00565940882594655)
(25.908491243639833, 0.00771803979503518)
(-58.90061791911219, -0.00339542777910609)
(-68.3259813506395, -0.00292706582698748)
(-76.18034021009562, 0.00262529271726657)
(-55.758786123065505, -0.00358673445619732)
(10.18565147964378, 0.0196118496056297)
(8.610377635965385, -0.0231887209384616)
(66.75509892653919, 0.00299594178340484)
(-77.75120283633034, -0.00257225428487172)
(63.613321321667165, 0.0031438984506466)
(62.04242549488138, -0.00322349592187255)
(55.758786123065505, -0.00358673445619732)
(-79.32206283663172, 0.00252131651220847)
(-63.613321321667165, 0.0031438984506466)
(-18.050311122187804, -0.0110758929204597)
(16.478194519911238, 0.0121316684745241)
(24.337072115977193, -0.00821618161670149)
(-62.04242549488138, -0.00322349592187255)
(96.60138616641379, -0.00207033593395571)
(99.74306033243167, -0.00200512683773813)
(88.7471755026564, 0.00225355708243497)
(51.04598323245382, 0.00391784805869645)
(-11.759726249344503, -0.0169918467910451)
(38.47801315516559, 0.00519733479481709)
(-54.18785982583734, 0.00369070650031279)
(-82.46377555970939, 0.0024252627583641)
(69.8968599047927, 0.00286128566232707)
(84.0346285545694, -0.00237992912409497)
(44.76211046520859, 0.00446778585366942)
(-91.88886446648316, 0.00217651007433517)
(47.90406939343085, 0.00417478325382633)
(-47.90406939343085, 0.00417478325382633)
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:
$$x_{1} = 74.6094747920599$$
$$x_{2} = 2.24670472895453$$
$$x_{3} = -93.4597065202651$$
$$x_{4} = 18.0503111221878$$
$$x_{5} = 77.7512028363303$$
$$x_{6} = -2.24670472895453$$
$$x_{7} = -24.3370721159772$$
$$x_{8} = -49.4750314121659$$
$$x_{9} = -99.7430603324317$$
$$x_{10} = 33.7647173885721$$
$$x_{11} = -46.3330961388114$$
$$x_{12} = 68.3259813506395$$
$$x_{13} = 90.3180208221014$$
$$x_{14} = -27.4798391439445$$
$$x_{15} = -652.665490738742$$
$$x_{16} = 30.6223651301872$$
$$x_{17} = -84.0346285545694$$
$$x_{18} = -40.0490643144726$$
$$x_{19} = -33.7647173885721$$
$$x_{20} = -5.45206082971445$$
$$x_{21} = 40.0490643144726$$
$$x_{22} = 11.7597262493445$$
$$x_{23} = 52.6169257678188$$
$$x_{24} = -90.3180208221014$$
$$x_{25} = -71.4677348441946$$
$$x_{26} = 46.3330961388114$$
$$x_{27} = -58.9006179191122$$
$$x_{28} = -68.3259813506395$$
$$x_{29} = -55.7587861230655$$
$$x_{30} = 8.61037763596538$$
$$x_{31} = -77.7512028363303$$
$$x_{32} = 62.0424254948814$$
$$x_{33} = 55.7587861230655$$
$$x_{34} = -18.0503111221878$$
$$x_{35} = 24.3370721159772$$
$$x_{36} = -62.0424254948814$$
$$x_{37} = 96.6013861664138$$
$$x_{38} = 99.7430603324317$$
$$x_{39} = -11.7597262493445$$
$$x_{40} = 84.0346285545694$$
Maxima of the function at points:
$$x_{40} = -3.86262591846885$$
$$x_{40} = 22.7655670069956$$
$$x_{40} = 60.4715244985757$$
$$x_{40} = 98.172223901556$$
$$x_{40} = -41.6200962353617$$
$$x_{40} = -60.4715244985757$$
$$x_{40} = 13.3330271294063$$
$$x_{40} = 76.1803402100956$$
$$x_{40} = 54.1878598258373$$
$$x_{40} = 82.4637755597094$$
$$x_{40} = -19.6222161805821$$
$$x_{40} = -32.1935597952787$$
$$x_{40} = -57.3297052975115$$
$$x_{40} = -98.172223901556$$
$$x_{40} = -85.6054794697228$$
$$x_{40} = -69.8968599047927$$
$$x_{40} = 204.987701063789$$
$$x_{40} = 32.1935597952787$$
$$x_{40} = -10.1856514796438$$
$$x_{40} = 41.6200962353617$$
$$x_{40} = 3.86262591846885$$
$$x_{40} = 85.6054794697228$$
$$x_{40} = -38.4780131551656$$
$$x_{40} = -25.9084912436398$$
$$x_{40} = -13.3330271294063$$
$$x_{40} = -16.4781945199112$$
$$x_{40} = 19.6222161805821$$
$$x_{40} = 91.8888644664832$$
$$x_{40} = -35.3358428558098$$
$$x_{40} = 25.9084912436398$$
$$x_{40} = -76.1803402100956$$
$$x_{40} = 10.1856514796438$$
$$x_{40} = 66.7550989265392$$
$$x_{40} = 63.6133213216672$$
$$x_{40} = -79.3220628366317$$
$$x_{40} = -63.6133213216672$$
$$x_{40} = 16.4781945199112$$
$$x_{40} = 88.7471755026564$$
$$x_{40} = 51.0459832324538$$
$$x_{40} = 38.4780131551656$$
$$x_{40} = -54.1878598258373$$
$$x_{40} = -82.4637755597094$$
$$x_{40} = 69.8968599047927$$
$$x_{40} = 44.7621104652086$$
$$x_{40} = -91.8888644664832$$
$$x_{40} = 47.9040693934309$$
$$x_{40} = -47.9040693934309$$
Decreasing at intervals
$$\left[99.7430603324317, \infty\right)$$
Increasing at intervals
$$\left(-\infty, -652.665490738742\right]$$