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$$2 \frac{1}{7 x} \cos{\left(2 x \right)} - \frac{\sin{\left(2 x \right)}}{7 x^{2}} = 0$$
Solve this equationThe roots of this equation
$$x_{1} = 41.6200962353617$$
$$x_{2} = -82.4637755597094$$
$$x_{3} = 66.7550989265392$$
$$x_{4} = 13.3330271294063$$
$$x_{5} = 11.7597262493445$$
$$x_{6} = -41.6200962353617$$
$$x_{7} = -54.1878598258373$$
$$x_{8} = -10.1856514796438$$
$$x_{9} = 204.987701063789$$
$$x_{10} = -46.3330961388114$$
$$x_{11} = 44.7621104652086$$
$$x_{12} = 84.0346285545694$$
$$x_{13} = -27.4798391439445$$
$$x_{14} = 3.86262591846885$$
$$x_{15} = 19.6222161805821$$
$$x_{16} = 77.7512028363303$$
$$x_{17} = -40.0490643144726$$
$$x_{18} = 18.0503111221878$$
$$x_{19} = 33.7647173885721$$
$$x_{20} = 2.24670472895453$$
$$x_{21} = -16.4781945199112$$
$$x_{22} = -55.7587861230655$$
$$x_{23} = -98.172223901556$$
$$x_{24} = -11.7597262493445$$
$$x_{25} = -60.4715244985757$$
$$x_{26} = 88.7471755026564$$
$$x_{27} = -90.3180208221014$$
$$x_{28} = -32.1935597952787$$
$$x_{29} = -57.3297052975115$$
$$x_{30} = 68.3259813506395$$
$$x_{31} = -69.8968599047927$$
$$x_{32} = 60.4715244985757$$
$$x_{33} = -68.3259813506395$$
$$x_{34} = 98.172223901556$$
$$x_{35} = -58.9006179191122$$
$$x_{36} = -77.7512028363303$$
$$x_{37} = -63.6133213216672$$
$$x_{38} = 24.3370721159772$$
$$x_{39} = 16.4781945199112$$
$$x_{40} = 46.3330961388114$$
$$x_{41} = -47.9040693934309$$
$$x_{42} = -85.6054794697228$$
$$x_{43} = 62.0424254948814$$
$$x_{44} = -18.0503111221878$$
$$x_{45} = -93.4597065202651$$
$$x_{46} = 82.4637755597094$$
$$x_{47} = 8.61037763596538$$
$$x_{48} = -79.3220628366317$$
$$x_{49} = 74.6094747920599$$
$$x_{50} = 63.6133213216672$$
$$x_{51} = 47.9040693934309$$
$$x_{52} = -99.7430603324317$$
$$x_{53} = 25.9084912436398$$
$$x_{54} = -25.9084912436398$$
$$x_{55} = 40.0490643144726$$
$$x_{56} = -5.45206082971445$$
$$x_{57} = -76.1803402100956$$
$$x_{58} = -19.6222161805821$$
$$x_{59} = -71.4677348441946$$
$$x_{60} = -84.0346285545694$$
$$x_{61} = -91.8888644664832$$
$$x_{62} = 55.7587861230655$$
$$x_{63} = -35.3358428558098$$
$$x_{64} = -13.3330271294063$$
$$x_{65} = -33.7647173885721$$
$$x_{66} = 38.4780131551656$$
$$x_{67} = -3.86262591846885$$
$$x_{68} = 91.8888644664832$$
$$x_{69} = 54.1878598258373$$
$$x_{70} = 32.1935597952787$$
$$x_{71} = 99.7430603324317$$
$$x_{72} = 85.6054794697228$$
$$x_{73} = -2.24670472895453$$
$$x_{74} = -49.4750314121659$$
$$x_{75} = -24.3370721159772$$
$$x_{76} = 10.1856514796438$$
$$x_{77} = 22.7655670069956$$
$$x_{78} = 52.6169257678188$$
$$x_{79} = 30.6223651301872$$
$$x_{80} = -62.0424254948814$$
$$x_{81} = 90.3180208221014$$
$$x_{82} = -38.4780131551656$$
$$x_{83} = 51.0459832324538$$
$$x_{84} = 69.8968599047927$$
$$x_{85} = 96.6013861664138$$
$$x_{86} = 76.1803402100956$$
The values of the extrema at the points:
(41.6200962353617, 0.00343216013772935)
(-82.46377555970939, 0.00173233054168864)
(66.75509892653919, 0.00213995841671774)
(13.333027129406338, 0.0107070057125518)
(11.759726249344503, -0.012137033422175)
(-41.6200962353617, 0.00343216013772935)
(-54.18785982583734, 0.00263621892879485)
(-10.18565147964378, 0.0140084640040212)
(204.98770106378876, 0.000696903849106796)
(-46.33309613881142, -0.00308308395700978)
(44.76211046520859, 0.00319127560976387)
(84.0346285545694, -0.00169994937435355)
(-27.479839143944467, -0.00519775609194375)
(3.8626259184688534, 0.0366784438645426)
(19.622216180582097, 0.0072780151693945)
(77.75120283633034, -0.00183732448919409)
(-40.04906431447256, -0.003566775234794)
(18.050311122187804, -0.00791135208604263)
(33.76471738857206, -0.00423049541329978)
(2.246704728954532, -0.0620667509174919)
(-16.478194519911238, 0.00866547748180294)
(-55.758786123065505, -0.00256195318299808)
(-98.172223901556, 0.00145514978055407)
(-11.759726249344503, -0.012137033422175)
(-60.47152449857575, 0.00236230624295931)
(88.7471755026564, 0.00160968363031069)
(-90.31802082210145, -0.00158168826685429)
(-32.19355979527871, 0.00443690965927467)
(-57.32970529751154, 0.00249175727077265)
(68.3259813506395, -0.00209076130499106)
(-69.8968599047927, 0.00204377547309076)
(60.47152449857575, 0.00236230624295931)
(-68.3259813506395, -0.00209076130499106)
(98.172223901556, 0.00145514978055407)
(-58.90061791911219, -0.00242530555650435)
(-77.75120283633034, -0.00183732448919409)
(-63.613321321667165, 0.00224564175046186)
(24.337072115977193, -0.00586870115478678)
(16.478194519911238, 0.00866547748180294)
(46.33309613881142, -0.00308308395700978)
(-47.90406939343085, 0.00298198803844738)
(-85.60547946972281, 0.00166875656869561)
(62.04242549488138, -0.00230249708705182)
(-18.050311122187804, -0.00791135208604263)
(-93.45970652026512, -0.00152852072655374)
(82.46377555970939, 0.00173233054168864)
(8.610377635965385, -0.0165633720989011)
(-79.32206283663172, 0.0018009403658632)
(74.60947479205991, -0.00191468892442683)
(63.613321321667165, 0.00224564175046186)
(47.90406939343085, 0.00298198803844738)
(-99.74306033243167, -0.00143223345552723)
(25.908491243639833, 0.00551288556788228)
(-25.908491243639833, 0.00551288556788228)
(40.04906431447256, -0.003566775234794)
(-5.4520608297144495, -0.0260929150923022)
(-76.18034021009562, 0.00187520908376183)
(-19.622216180582097, 0.0072780151693945)
(-71.46773484419464, -0.0019988551075779)
(-84.0346285545694, -0.00169994937435355)
(-91.88886446648316, 0.00155465005309655)
(55.758786123065505, -0.00256195318299808)
(-35.33584285580975, 0.00404243487567611)
(-13.333027129406338, 0.0107070057125518)
(-33.76471738857206, -0.00423049541329978)
(38.47801315516559, 0.00371238199629792)
(-3.8626259184688534, 0.0366784438645426)
(91.88886446648316, 0.00155465005309655)
(54.18785982583734, 0.00263621892879485)
(32.19355979527871, 0.00443690965927467)
(99.74306033243167, -0.00143223345552723)
(85.60547946972281, 0.00166875656869561)
(-2.246704728954532, -0.0620667509174919)
(-49.47503141216594, -0.00288731192471544)
(-24.337072115977193, -0.00586870115478678)
(10.18565147964378, 0.0140084640040212)
(22.76556700699564, 0.00627362806528069)
(52.6169257678188, -0.00271491903393815)
(30.6223651301872, -0.00466450266314224)
(-62.04242549488138, -0.00230249708705182)
(90.31802082210145, -0.00158168826685429)
(-38.47801315516559, 0.00371238199629792)
(51.04598323245382, 0.0027984628990689)
(69.8968599047927, 0.00204377547309076)
(96.60138616641379, -0.00147881138139694)
(76.18034021009562, 0.00187520908376183)
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:
$$x_{1} = 11.7597262493445$$
$$x_{2} = -46.3330961388114$$
$$x_{3} = 84.0346285545694$$
$$x_{4} = -27.4798391439445$$
$$x_{5} = 77.7512028363303$$
$$x_{6} = -40.0490643144726$$
$$x_{7} = 18.0503111221878$$
$$x_{8} = 33.7647173885721$$
$$x_{9} = 2.24670472895453$$
$$x_{10} = -55.7587861230655$$
$$x_{11} = -11.7597262493445$$
$$x_{12} = -90.3180208221014$$
$$x_{13} = 68.3259813506395$$
$$x_{14} = -68.3259813506395$$
$$x_{15} = -58.9006179191122$$
$$x_{16} = -77.7512028363303$$
$$x_{17} = 24.3370721159772$$
$$x_{18} = 46.3330961388114$$
$$x_{19} = 62.0424254948814$$
$$x_{20} = -18.0503111221878$$
$$x_{21} = -93.4597065202651$$
$$x_{22} = 8.61037763596538$$
$$x_{23} = 74.6094747920599$$
$$x_{24} = -99.7430603324317$$
$$x_{25} = 40.0490643144726$$
$$x_{26} = -5.45206082971445$$
$$x_{27} = -71.4677348441946$$
$$x_{28} = -84.0346285545694$$
$$x_{29} = 55.7587861230655$$
$$x_{30} = -33.7647173885721$$
$$x_{31} = 99.7430603324317$$
$$x_{32} = -2.24670472895453$$
$$x_{33} = -49.4750314121659$$
$$x_{34} = -24.3370721159772$$
$$x_{35} = 52.6169257678188$$
$$x_{36} = 30.6223651301872$$
$$x_{37} = -62.0424254948814$$
$$x_{38} = 90.3180208221014$$
$$x_{39} = 96.6013861664138$$
Maxima of the function at points:
$$x_{39} = 41.6200962353617$$
$$x_{39} = -82.4637755597094$$
$$x_{39} = 66.7550989265392$$
$$x_{39} = 13.3330271294063$$
$$x_{39} = -41.6200962353617$$
$$x_{39} = -54.1878598258373$$
$$x_{39} = -10.1856514796438$$
$$x_{39} = 204.987701063789$$
$$x_{39} = 44.7621104652086$$
$$x_{39} = 3.86262591846885$$
$$x_{39} = 19.6222161805821$$
$$x_{39} = -16.4781945199112$$
$$x_{39} = -98.172223901556$$
$$x_{39} = -60.4715244985757$$
$$x_{39} = 88.7471755026564$$
$$x_{39} = -32.1935597952787$$
$$x_{39} = -57.3297052975115$$
$$x_{39} = -69.8968599047927$$
$$x_{39} = 60.4715244985757$$
$$x_{39} = 98.172223901556$$
$$x_{39} = -63.6133213216672$$
$$x_{39} = 16.4781945199112$$
$$x_{39} = -47.9040693934309$$
$$x_{39} = -85.6054794697228$$
$$x_{39} = 82.4637755597094$$
$$x_{39} = -79.3220628366317$$
$$x_{39} = 63.6133213216672$$
$$x_{39} = 47.9040693934309$$
$$x_{39} = 25.9084912436398$$
$$x_{39} = -25.9084912436398$$
$$x_{39} = -76.1803402100956$$
$$x_{39} = -19.6222161805821$$
$$x_{39} = -91.8888644664832$$
$$x_{39} = -35.3358428558098$$
$$x_{39} = -13.3330271294063$$
$$x_{39} = 38.4780131551656$$
$$x_{39} = -3.86262591846885$$
$$x_{39} = 91.8888644664832$$
$$x_{39} = 54.1878598258373$$
$$x_{39} = 32.1935597952787$$
$$x_{39} = 85.6054794697228$$
$$x_{39} = 10.1856514796438$$
$$x_{39} = 22.7655670069956$$
$$x_{39} = -38.4780131551656$$
$$x_{39} = 51.0459832324538$$
$$x_{39} = 69.8968599047927$$
$$x_{39} = 76.1803402100956$$
Decreasing at intervals
$$\left[99.7430603324317, \infty\right)$$
Increasing at intervals
$$\left(-\infty, -99.7430603324317\right]$$