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$$9 \frac{1}{3 x} \cos{\left(9 x \right)} - \frac{\sin{\left(9 x \right)}}{3 x^{2}} = 0$$
Solve this equationThe roots of this equation
$$x_{1} = -43.8074824089874$$
$$x_{2} = -78.0160593188984$$
$$x_{3} = -95.8184470900558$$
$$x_{4} = -38.5714563970313$$
$$x_{5} = 54.2795122909533$$
$$x_{6} = 96.1675134081304$$
$$x_{7} = -71.7328601507786$$
$$x_{8} = 80.110458558433$$
$$x_{9} = -56.0248486285328$$
$$x_{10} = -23.9104944271503$$
$$x_{11} = 94.0731154476223$$
$$x_{12} = 14.1362936270371$$
$$x_{13} = -75.9216598511068$$
$$x_{14} = 90.2333855087611$$
$$x_{15} = 86.0445886433835$$
$$x_{16} = 22.1651245183181$$
$$x_{17} = -63.0061900532919$$
$$x_{18} = -27.7502902244063$$
$$x_{19} = 28.0993616014094$$
$$x_{20} = -45.9018903700114$$
$$x_{21} = -59.8645871168961$$
$$x_{22} = 60.2136541628164$$
$$x_{23} = -26.0049311138353$$
$$x_{24} = 20.0706712938003$$
$$x_{25} = 24.2595677068976$$
$$x_{26} = 12.0417466279639$$
$$x_{27} = 52.1851080602357$$
$$x_{28} = 6.10663092087655$$
$$x_{29} = -9.94713565893524$$
$$x_{30} = -79.7613920335987$$
$$x_{31} = 74.1763267729682$$
$$x_{32} = -47.996297208815$$
$$x_{33} = -93.724049108452$$
$$x_{34} = -83.950189961441$$
$$x_{35} = -13.7872056655292$$
$$x_{36} = 26.0049311138353$$
$$x_{37} = 4.01118024937507$$
$$x_{38} = -35.7789046122982$$
$$x_{39} = 88.1389871552083$$
$$x_{40} = -82.2048575871571$$
$$x_{41} = -8.89979206988279$$
$$x_{42} = 8.20154228896452$$
$$x_{43} = -69.9875266068451$$
$$x_{44} = 66.147792012951$$
$$x_{45} = -57.7701845379769$$
$$x_{46} = 64.0533908079408$$
$$x_{47} = 58.1192516718838$$
$$x_{48} = 82.2048575871571$$
$$x_{49} = -15.8817188542655$$
$$x_{50} = -87.7899207478679$$
$$x_{51} = -4.01118024937507$$
$$x_{52} = 63.0061900532919$$
$$x_{53} = 72.0819268346241$$
$$x_{54} = 42.0621414632736$$
$$x_{55} = -19.7215945561459$$
$$x_{56} = 78.0160593188984$$
$$x_{57} = 44.1565504872002$$
$$x_{58} = 30.1937871799132$$
$$x_{59} = 56.0248486285328$$
$$x_{60} = -67.8931260628453$$
$$x_{61} = 34.0335576650028$$
$$x_{62} = 39.9677309803002$$
$$x_{63} = 50.0907030661665$$
$$x_{64} = 11.3435518294342$$
$$x_{65} = -5.75744249858663$$
$$x_{66} = -39.9677309803002$$
$$x_{67} = 46.2509582502934$$
$$x_{68} = -37.873318796221$$
$$x_{69} = -53.9304449684033$$
$$x_{70} = -97.9128449484844$$
$$x_{71} = -1.91341725243675$$
$$x_{72} = -7.85240952351328$$
$$x_{73} = -65.7987251724283$$
$$x_{74} = 16.2308014226856$$
$$x_{75} = -61.9589891903567$$
$$x_{76} = -34.0335576650028$$
$$x_{77} = -73.8272601356318$$
$$x_{78} = 17.9762045253295$$
$$x_{79} = -49.7416354861698$$
$$x_{80} = -29.844716547209$$
$$x_{81} = -89.8843191270239$$
$$x_{82} = 91.978717356953$$
$$x_{83} = 2.26347810658751$$
$$x_{84} = 100.007242691477$$
$$x_{85} = 38.2223876235673$$
$$x_{86} = -100.007242691477$$
$$x_{87} = -21.8160497559013$$
$$x_{88} = 98.2619112468002$$
$$x_{89} = 68.242192843374$$
$$x_{90} = -17.9762045253295$$
$$x_{91} = -91.978717356953$$
$$x_{92} = 76.2707264457212$$
$$x_{93} = 69.9875266068451$$
$$x_{94} = -31.9391387754254$$
$$x_{95} = -51.8360406167335$$
$$x_{96} = -81.8557910963226$$
$$x_{97} = 47.996297208815$$
$$x_{98} = 36.1279737965903$$
The values of the extrema at the points:
(-43.80748240898745, -0.0076090257378385)
(-78.0160593188984, -0.00427262025514283)
(-95.81844709005577, 0.0034787989092339)
(-38.57145639703134, 0.00864193321819759)
(54.27951229095335, -0.00614103960935887)
(96.16751340813036, -0.00346617167307549)
(-71.7328601507786, -0.00464686522683571)
(80.11045855843302, -0.00416091754704642)
(-56.024848628532794, 0.00594972919963856)
(-23.91049442715029, 0.0139407294720594)
(94.07311544762229, -0.00354334072218502)
(14.136293627037148, 0.0235792383798495)
(-75.92165985110675, -0.00439048589054966)
(90.23338550876106, 0.00369412140240821)
(86.04458864338353, 0.00387395721998559)
(22.165124518318148, -0.0150384512830359)
(-63.006190053291924, 0.00529047724887677)
(-27.750290224406303, -0.0120117901008536)
(28.099361601409353, 0.0118625729692271)
(-45.90189037001142, -0.00726184377337051)
(-59.86458711689614, -0.00556811255602698)
(60.21365416281644, 0.00553583353242669)
(-26.004931113835333, 0.0128179647645914)
(20.07067129380032, -0.0166077268019701)
(24.259567706897627, -0.0137401391980584)
(12.041746627963855, 0.0276802987523459)
(52.18510806023566, -0.00638750383324617)
(6.106630920876548, -0.0545764389654094)
(-9.947135658935244, 0.0335083939025207)
(-79.76139203359868, 0.00417912728710772)
(74.17632677296824, 0.00449379167004811)
(-47.99629720881502, -0.00694496158084112)
(-93.72404910845204, 0.00355653754040813)
(-83.95018996144104, 0.003970604968599)
(-13.787205665529196, -0.0241762194140442)
(26.004931113835333, 0.0128179647645914)
(4.011180249375068, -0.0830691969034476)
(-35.77890461229816, 0.00931643183629947)
(88.13898715520828, 0.00378190264292246)
(-82.2048575871571, -0.00405490671270037)
(-8.89979206988279, -0.037451139965337)
(8.201542288964516, -0.0406390330354199)
(-69.98752660684514, 0.00476274744121016)
(66.147792012951, -0.00503921375054354)
(-57.770184537976945, -0.00576997839747367)
(64.05339080794077, -0.00520398417039424)
(58.119251671883795, 0.00573532374552275)
(82.2048575871571, -0.00405490671270037)
(-15.881718854265474, -0.0209879786295679)
(-87.78992074786785, -0.00379694005321676)
(-4.011180249375068, -0.0830691969034476)
(63.006190053291924, 0.00529047724887677)
(72.08192683462414, 0.00462436219392993)
(42.06214146327362, 0.0079247551061351)
(-19.721594556145867, 0.0169016781182623)
(78.0160593188984, -0.00427262025514283)
(44.15655048720022, 0.00754887495437416)
(30.193787179913155, 0.0110397239798272)
(56.024848628532794, 0.00594972919963856)
(-67.89312606284533, 0.00490967062907198)
(34.033557665002824, -0.00979420253958615)
(39.96773098030021, 0.00834002924557917)
(50.09070306616655, -0.00665457845198908)
(11.343551829434183, 0.0293838604402234)
(-5.75744249858663, 0.0578852984627639)
(-39.96773098030021, 0.00834002924557917)
(46.250958250293436, 0.00720703708776362)
(-37.873318796221014, 0.00880123288502446)
(-53.93044496840333, 0.00618078760672051)
(-97.9128449484844, 0.00340438600146795)
(-1.913417252436752, -0.173915407038462)
(-7.852409523513278, 0.0424455661945991)
(-65.7987251724283, 0.00506594705601915)
(16.230801422685595, 0.0205366028662432)
(-61.95898919035668, -0.00537989405092629)
(-34.033557665002824, -0.00979420253958615)
(-73.82726013563183, -0.00451503896000007)
(17.976204525329518, -0.0185426776583483)
(-49.741635486169756, 0.00670127748034431)
(-29.84471654720901, -0.0111688453378519)
(-89.88431912702386, -0.00370846752682146)
(91.97871735695301, -0.00362402411881582)
(2.263478106587507, 0.147088872042222)
(100.0072426914767, 0.00333308987060412)
(38.22238762356733, -0.00872085564665135)
(-100.0072426914767, 0.00333308987060412)
(-21.816049755901332, 0.0152790726958178)
(98.26191124680018, -0.00339229225240162)
(68.242192843374, -0.00488455715757307)
(-17.976204525329518, -0.0185426776583483)
(-91.97871735695301, -0.00362402411881582)
(76.27072644572121, 0.00437039209087127)
(69.98752660684514, 0.00476274744121016)
(-31.93913877542537, -0.0104364528624995)
(-51.8360406167335, 0.00643051752404291)
(-81.85579109632265, 0.00407219845755388)
(47.99629720881502, -0.00694496158084112)
(36.12797379659035, -0.00922641714657509)
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:
$$x_{1} = -43.8074824089874$$
$$x_{2} = -78.0160593188984$$
$$x_{3} = 54.2795122909533$$
$$x_{4} = 96.1675134081304$$
$$x_{5} = -71.7328601507786$$
$$x_{6} = 80.110458558433$$
$$x_{7} = 94.0731154476223$$
$$x_{8} = -75.9216598511068$$
$$x_{9} = 22.1651245183181$$
$$x_{10} = -27.7502902244063$$
$$x_{11} = -45.9018903700114$$
$$x_{12} = -59.8645871168961$$
$$x_{13} = 20.0706712938003$$
$$x_{14} = 24.2595677068976$$
$$x_{15} = 52.1851080602357$$
$$x_{16} = 6.10663092087655$$
$$x_{17} = -47.996297208815$$
$$x_{18} = -13.7872056655292$$
$$x_{19} = 4.01118024937507$$
$$x_{20} = -82.2048575871571$$
$$x_{21} = -8.89979206988279$$
$$x_{22} = 8.20154228896452$$
$$x_{23} = 66.147792012951$$
$$x_{24} = -57.7701845379769$$
$$x_{25} = 64.0533908079408$$
$$x_{26} = 82.2048575871571$$
$$x_{27} = -15.8817188542655$$
$$x_{28} = -87.7899207478679$$
$$x_{29} = -4.01118024937507$$
$$x_{30} = 78.0160593188984$$
$$x_{31} = 34.0335576650028$$
$$x_{32} = 50.0907030661665$$
$$x_{33} = -1.91341725243675$$
$$x_{34} = -61.9589891903567$$
$$x_{35} = -34.0335576650028$$
$$x_{36} = -73.8272601356318$$
$$x_{37} = 17.9762045253295$$
$$x_{38} = -29.844716547209$$
$$x_{39} = -89.8843191270239$$
$$x_{40} = 91.978717356953$$
$$x_{41} = 38.2223876235673$$
$$x_{42} = 98.2619112468002$$
$$x_{43} = 68.242192843374$$
$$x_{44} = -17.9762045253295$$
$$x_{45} = -91.978717356953$$
$$x_{46} = -31.9391387754254$$
$$x_{47} = 47.996297208815$$
$$x_{48} = 36.1279737965903$$
Maxima of the function at points:
$$x_{48} = -95.8184470900558$$
$$x_{48} = -38.5714563970313$$
$$x_{48} = -56.0248486285328$$
$$x_{48} = -23.9104944271503$$
$$x_{48} = 14.1362936270371$$
$$x_{48} = 90.2333855087611$$
$$x_{48} = 86.0445886433835$$
$$x_{48} = -63.0061900532919$$
$$x_{48} = 28.0993616014094$$
$$x_{48} = 60.2136541628164$$
$$x_{48} = -26.0049311138353$$
$$x_{48} = 12.0417466279639$$
$$x_{48} = -9.94713565893524$$
$$x_{48} = -79.7613920335987$$
$$x_{48} = 74.1763267729682$$
$$x_{48} = -93.724049108452$$
$$x_{48} = -83.950189961441$$
$$x_{48} = 26.0049311138353$$
$$x_{48} = -35.7789046122982$$
$$x_{48} = 88.1389871552083$$
$$x_{48} = -69.9875266068451$$
$$x_{48} = 58.1192516718838$$
$$x_{48} = 63.0061900532919$$
$$x_{48} = 72.0819268346241$$
$$x_{48} = 42.0621414632736$$
$$x_{48} = -19.7215945561459$$
$$x_{48} = 44.1565504872002$$
$$x_{48} = 30.1937871799132$$
$$x_{48} = 56.0248486285328$$
$$x_{48} = -67.8931260628453$$
$$x_{48} = 39.9677309803002$$
$$x_{48} = 11.3435518294342$$
$$x_{48} = -5.75744249858663$$
$$x_{48} = -39.9677309803002$$
$$x_{48} = 46.2509582502934$$
$$x_{48} = -37.873318796221$$
$$x_{48} = -53.9304449684033$$
$$x_{48} = -97.9128449484844$$
$$x_{48} = -7.85240952351328$$
$$x_{48} = -65.7987251724283$$
$$x_{48} = 16.2308014226856$$
$$x_{48} = -49.7416354861698$$
$$x_{48} = 2.26347810658751$$
$$x_{48} = 100.007242691477$$
$$x_{48} = -100.007242691477$$
$$x_{48} = -21.8160497559013$$
$$x_{48} = 76.2707264457212$$
$$x_{48} = 69.9875266068451$$
$$x_{48} = -51.8360406167335$$
$$x_{48} = -81.8557910963226$$
Decreasing at intervals
$$\left[98.2619112468002, \infty\right)$$
Increasing at intervals
$$\left(-\infty, -91.978717356953\right]$$