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$$7 \frac{1}{6 x} \cos{\left(7 x \right)} - \frac{\sin{\left(7 x \right)}}{6 x^{2}} = 0$$
Solve this equationThe roots of this equation
$$x_{1} = -50.0406751517951$$
$$x_{2} = -43.7574312835751$$
$$x_{3} = -79.6615575304586$$
$$x_{4} = 19.9705314013693$$
$$x_{5} = 100.306361981441$$
$$x_{6} = 80.1103579161908$$
$$x_{7} = -63.9535313393611$$
$$x_{8} = -13.6868769695517$$
$$x_{9} = -21.7658114885988$$
$$x_{10} = 6.05541622401885$$
$$x_{11} = 8.30032210778509$$
$$x_{12} = 173.011877464559$$
$$x_{13} = -11.8914560672462$$
$$x_{14} = -17.7264072842518$$
$$x_{15} = -95.8183629466279$$
$$x_{16} = 15.9310817494473$$
$$x_{17} = 18.1752346633335$$
$$x_{18} = -6.05541622401885$$
$$x_{19} = 48.2454641744854$$
$$x_{20} = -7.85138261255556$$
$$x_{21} = -72.031948236685$$
$$x_{22} = 28.049206829016$$
$$x_{23} = 96.2671628900941$$
$$x_{24} = 88.1887623614173$$
$$x_{25} = 2.00945627326164$$
$$x_{26} = -65.748735854334$$
$$x_{27} = 78.3151562745643$$
$$x_{28} = -25.8051488063147$$
$$x_{29} = 36.1277506303835$$
$$x_{30} = -77.8663558220856$$
$$x_{31} = 26.253961276138$$
$$x_{32} = -15.9310817494473$$
$$x_{33} = 58.1191129483684$$
$$x_{34} = -98.0623625730701$$
$$x_{35} = -87.7399622271952$$
$$x_{36} = -73.8271509280264$$
$$x_{37} = 30.2932554779114$$
$$x_{38} = 10.0959551016599$$
$$x_{39} = 52.2846874079873$$
$$x_{40} = 84.1495606988712$$
$$x_{41} = 4.25879982727042$$
$$x_{42} = 32.0884889694752$$
$$x_{43} = -29.8444463965395$$
$$x_{44} = 85.9447615666809$$
$$x_{45} = -341.760341100684$$
$$x_{46} = 89.9839627802295$$
$$x_{47} = -33.883718467298$$
$$x_{48} = -83.7007604479657$$
$$x_{49} = 63.9535313393611$$
$$x_{50} = -99.8575621165067$$
$$x_{51} = 41.9622155286474$$
$$x_{52} = 74.2759515488275$$
$$x_{53} = -41.9622155286474$$
$$x_{54} = -69.7879443740829$$
$$x_{55} = -67.9927408510451$$
$$x_{56} = 56.7727077692574$$
$$x_{57} = 67.9927408510451$$
$$x_{58} = -2.00945627326164$$
$$x_{59} = 46.0014487872405$$
$$x_{60} = -56.3239059544124$$
$$x_{61} = -46.0014487872405$$
$$x_{62} = -61.7095249827622$$
$$x_{63} = -39.7181932985428$$
$$x_{64} = 62.1583263211096$$
$$x_{65} = -72.4807489415142$$
$$x_{66} = -81.9055593018343$$
$$x_{67} = 22.2146293818087$$
$$x_{68} = -55.8751040935535$$
$$x_{69} = -85.9447615666809$$
$$x_{70} = -59.9143192716405$$
$$x_{71} = -37.9229731731142$$
$$x_{72} = -94.0231630779702$$
$$x_{73} = 76.0711538349461$$
$$x_{74} = 70.2367451931726$$
$$x_{75} = 24.0098938727341$$
$$x_{76} = -28.049206829016$$
$$x_{77} = -19.9705314013693$$
$$x_{78} = 54.0798961670661$$
$$x_{79} = -76.0711538349461$$
$$x_{80} = 98.0623625730701$$
$$x_{81} = 50.0406751517951$$
$$x_{82} = -91.779163018434$$
$$x_{83} = 44.2062349690925$$
$$x_{84} = -47.7966612520415$$
$$x_{85} = -51.835885077988$$
$$x_{86} = 72.031948236685$$
$$x_{87} = -3.80943632268752$$
$$x_{88} = 92.2279630510011$$
$$x_{89} = -89.9839627802295$$
$$x_{90} = 37.4741677776243$$
$$x_{91} = 59.0167161900147$$
$$x_{92} = 1728.10034714005$$
$$x_{93} = 94.0231630779702$$
$$x_{94} = 40.1669979901791$$
$$x_{95} = 66.1975369092382$$
$$x_{96} = -24.0098938727341$$
$$x_{97} = 14.1357232606188$$
The values of the extrema at the points:
(-50.040675151795064, -0.00333061028850224)
(-43.75743128357506, -0.00380885654317239)
(-79.6615575304586, -0.00209218101980467)
(19.970531401369342, 0.00834541651512062)
(100.3063619814408, -0.00166157454366666)
(80.1103579161908, 0.00208046008035556)
(-63.95353133936111, 0.00260605235346723)
(-13.68687696955167, 0.0121764511569935)
(-21.76581148859875, 0.00765710375869415)
(6.055416224018845, -0.0275159129338524)
(8.300322107785085, 0.0200765687168993)
(173.01187746455872, -0.000963324670497339)
(-11.891456067246201, 0.0140146538957282)
(-17.726407284251824, -0.00940186310546161)
(-95.81836294662794, -0.00173940021834752)
(15.931081749447287, -0.0104613088305755)
(18.175234663333477, 0.00916970381438593)
(-6.055416224018845, -0.0275159129338524)
(48.245464174485434, -0.00345454104075334)
(-7.851382612555562, -0.0212241707087703)
(-72.03194823668503, 0.00231378357763722)
(28.049206829015997, 0.00594186160392913)
(96.2671628900941, 0.00173129110852122)
(88.18876236141728, 0.0018898830591425)
(2.0094562732616392, 0.0827323693588725)
(-65.74873585433403, 0.00253489699977008)
(78.31515627456432, 0.00212814986661164)
(-25.805148806314698, -0.00645856042298836)
(36.12775063038345, 0.00461322282136609)
(-77.86635582208555, -0.00214041590124719)
(26.253961276138043, 0.00634815438347758)
(-15.931081749447287, -0.0104613088305755)
(58.11911294836839, -0.00286766529513478)
(-98.06236257307013, 0.00169959692422771)
(-87.73996222719515, -0.00189955000572286)
(-73.82715092802644, 0.00225752114969546)
(30.293255477911444, -0.00550171352771523)
(10.095955101659928, 0.0165066090756774)
(52.28468740798735, 0.00318766454985892)
(84.14956069887116, -0.00198059770143643)
(4.258799827270423, -0.0391126575249652)
(32.088488969475215, -0.00519391907722712)
(-29.844446396539475, 0.00558444794353813)
(85.94476156668094, -0.00193922739894047)
(-341.7603411006841, -0.000487671131089389)
(89.98396278022946, 0.00185217956047062)
(-33.883718467298024, -0.00491873952841744)
(-83.70076044796572, 0.00199121755910715)
(63.95353133936111, 0.00260605235346723)
(-99.85756211650671, 0.00166904230967432)
(41.962215528647384, -0.00397180412744887)
(74.27595154882749, -0.00224388048790643)
(-41.962215528647384, -0.00397180412744887)
(-69.78794437408288, -0.00238818207030623)
(-67.99274085104514, -0.00245123665716453)
(56.77270776925742, 0.00293567359336773)
(67.99274085104514, -0.00245123665716453)
(-2.0094562732616392, 0.0827323693588725)
(46.0014487872405, 0.00362305682522757)
(-56.32390595441243, -0.00295906556471497)
(-46.0014487872405, 0.00362305682522757)
(-61.709524982762176, -0.00270081839254932)
(-39.71819329854276, 0.00419620266618035)
(62.158326321109584, 0.00268131779536268)
(-72.48074894151424, -0.00229945668851025)
(-81.90555930183429, 0.00203486081503426)
(22.214629381808667, -0.00750240833087585)
(-55.87510409355348, 0.00298283331435769)
(-85.94476156668094, -0.00193922739894047)
(-59.914319271640544, -0.00278174224344856)
(-37.92297317311419, 0.00439484223382984)
(-94.02316307797024, -0.00177261079965872)
(76.07115383494614, -0.00219092736703475)
(70.23674519317257, 0.00237292205764339)
(24.009893872734114, -0.00694145994527699)
(-28.049206829015997, 0.00594186160392913)
(-19.970531401369342, 0.00834541651512062)
(54.07989616706608, 0.00308184920794143)
(-76.07115383494614, -0.00219092736703475)
(98.06236257307013, 0.00169959692422771)
(50.040675151795064, -0.00333061028850224)
(-91.77916301843398, 0.00181595102076066)
(44.20623496909249, 0.00377018754292901)
(-47.79666125204154, 0.00348697833424961)
(-51.835885077988, -0.00321526358585061)
(72.03194823668503, 0.00231378357763722)
(-3.809436322687525, 0.043720273326253)
(92.22796305100107, -0.00180711425488056)
(-89.98396278022946, 0.00185217956047062)
(37.47416777762428, -0.00444747583530982)
(59.016716190014684, -0.00282405035631075)
(1728.1003471400488, 9.64450162706156e-5)
(94.02316307797024, -0.00177261079965872)
(40.16699799017913, -0.00414931712378136)
(66.19753690923824, -0.00251771117708913)
(-24.009893872734114, -0.00694145994527699)
(14.13572326061884, -0.0117898570259214)
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:
$$x_{1} = -50.0406751517951$$
$$x_{2} = -43.7574312835751$$
$$x_{3} = -79.6615575304586$$
$$x_{4} = 100.306361981441$$
$$x_{5} = 6.05541622401885$$
$$x_{6} = 173.011877464559$$
$$x_{7} = -17.7264072842518$$
$$x_{8} = -95.8183629466279$$
$$x_{9} = 15.9310817494473$$
$$x_{10} = -6.05541622401885$$
$$x_{11} = 48.2454641744854$$
$$x_{12} = -7.85138261255556$$
$$x_{13} = -25.8051488063147$$
$$x_{14} = -77.8663558220856$$
$$x_{15} = -15.9310817494473$$
$$x_{16} = 58.1191129483684$$
$$x_{17} = -87.7399622271952$$
$$x_{18} = 30.2932554779114$$
$$x_{19} = 84.1495606988712$$
$$x_{20} = 4.25879982727042$$
$$x_{21} = 32.0884889694752$$
$$x_{22} = 85.9447615666809$$
$$x_{23} = -341.760341100684$$
$$x_{24} = -33.883718467298$$
$$x_{25} = 41.9622155286474$$
$$x_{26} = 74.2759515488275$$
$$x_{27} = -41.9622155286474$$
$$x_{28} = -69.7879443740829$$
$$x_{29} = -67.9927408510451$$
$$x_{30} = 67.9927408510451$$
$$x_{31} = -56.3239059544124$$
$$x_{32} = -61.7095249827622$$
$$x_{33} = -72.4807489415142$$
$$x_{34} = 22.2146293818087$$
$$x_{35} = -85.9447615666809$$
$$x_{36} = -59.9143192716405$$
$$x_{37} = -94.0231630779702$$
$$x_{38} = 76.0711538349461$$
$$x_{39} = 24.0098938727341$$
$$x_{40} = -76.0711538349461$$
$$x_{41} = 50.0406751517951$$
$$x_{42} = -51.835885077988$$
$$x_{43} = 92.2279630510011$$
$$x_{44} = 37.4741677776243$$
$$x_{45} = 59.0167161900147$$
$$x_{46} = 94.0231630779702$$
$$x_{47} = 40.1669979901791$$
$$x_{48} = 66.1975369092382$$
$$x_{49} = -24.0098938727341$$
$$x_{50} = 14.1357232606188$$
Maxima of the function at points:
$$x_{50} = 19.9705314013693$$
$$x_{50} = 80.1103579161908$$
$$x_{50} = -63.9535313393611$$
$$x_{50} = -13.6868769695517$$
$$x_{50} = -21.7658114885988$$
$$x_{50} = 8.30032210778509$$
$$x_{50} = -11.8914560672462$$
$$x_{50} = 18.1752346633335$$
$$x_{50} = -72.031948236685$$
$$x_{50} = 28.049206829016$$
$$x_{50} = 96.2671628900941$$
$$x_{50} = 88.1887623614173$$
$$x_{50} = 2.00945627326164$$
$$x_{50} = -65.748735854334$$
$$x_{50} = 78.3151562745643$$
$$x_{50} = 36.1277506303835$$
$$x_{50} = 26.253961276138$$
$$x_{50} = -98.0623625730701$$
$$x_{50} = -73.8271509280264$$
$$x_{50} = 10.0959551016599$$
$$x_{50} = 52.2846874079873$$
$$x_{50} = -29.8444463965395$$
$$x_{50} = 89.9839627802295$$
$$x_{50} = -83.7007604479657$$
$$x_{50} = 63.9535313393611$$
$$x_{50} = -99.8575621165067$$
$$x_{50} = 56.7727077692574$$
$$x_{50} = -2.00945627326164$$
$$x_{50} = 46.0014487872405$$
$$x_{50} = -46.0014487872405$$
$$x_{50} = -39.7181932985428$$
$$x_{50} = 62.1583263211096$$
$$x_{50} = -81.9055593018343$$
$$x_{50} = -55.8751040935535$$
$$x_{50} = -37.9229731731142$$
$$x_{50} = 70.2367451931726$$
$$x_{50} = -28.049206829016$$
$$x_{50} = -19.9705314013693$$
$$x_{50} = 54.0798961670661$$
$$x_{50} = 98.0623625730701$$
$$x_{50} = -91.779163018434$$
$$x_{50} = 44.2062349690925$$
$$x_{50} = -47.7966612520415$$
$$x_{50} = 72.031948236685$$
$$x_{50} = -3.80943632268752$$
$$x_{50} = -89.9839627802295$$
$$x_{50} = 1728.10034714005$$
Decreasing at intervals
$$\left[173.011877464559, \infty\right)$$
Increasing at intervals
$$\left(-\infty, -341.760341100684\right]$$