Para hallar los extremos hay que resolver la ecuación
$$\frac{d}{d x} f{\left(x \right)} = 0$$
(la derivada es igual a cero),
y las raíces de esta ecuación serán los extremos de esta función:
$$\frac{d}{d x} f{\left(x \right)} = $$
primera derivada$$4 \frac{1}{5 x} \cos{\left(4 x \right)} - \frac{\sin{\left(4 x \right)}}{5 x^{2}} = 0$$
Resolvermos esta ecuaciónRaíces de esta ecuación
$$x_{1} = -13.7399195719722$$
$$x_{2} = -43.5881642087982$$
$$x_{3} = 40.4464601819914$$
$$x_{4} = 71.8630622447071$$
$$x_{5} = 44.3735877513282$$
$$x_{6} = 70.2922464828135$$
$$x_{7} = 18.4534701501702$$
$$x_{8} = 74.2192843450433$$
$$x_{9} = 82.0733465634182$$
$$x_{10} = 93.8544146025812$$
$$x_{11} = -12.1685360579886$$
$$x_{12} = -60.0819192595118$$
$$x_{13} = 394.662418744048$$
$$x_{14} = -16.0967798976394$$
$$x_{15} = 45.9444322332416$$
$$x_{16} = -42.0173142772847$$
$$x_{17} = 23.9520346967154$$
$$x_{18} = -71.8630622447071$$
$$x_{19} = -27.8793930615328$$
$$x_{20} = 12.1685360579886$$
$$x_{21} = 16.0967798976394$$
$$x_{22} = -9.81110809029105$$
$$x_{23} = 75.0046913263314$$
$$x_{24} = -100.13764169355$$
$$x_{25} = 31.8066606608336$$
$$x_{26} = -83.6441571910727$$
$$x_{27} = -78.146317479006$$
$$x_{28} = 1.93131295923443$$
$$x_{29} = 8.23909725995562$$
$$x_{30} = -59.2965073177715$$
$$x_{31} = -144.905529831764$$
$$x_{32} = -79.7171295652025$$
$$x_{33} = -20.0245321572363$$
$$x_{34} = 64.0089738962685$$
$$x_{35} = -56.1548556965816$$
$$x_{36} = 56.1548556965816$$
$$x_{37} = 62.4381530049142$$
$$x_{38} = 67.9360211548046$$
$$x_{39} = 53.7986124610881$$
$$x_{40} = -87.5711815156414$$
$$x_{41} = 4.30518881798269$$
$$x_{42} = 49.8715301662158$$
$$x_{43} = -75.7900981248612$$
$$x_{44} = -67.9360211548046$$
$$x_{45} = -68.7214298324352$$
$$x_{46} = -35.7338674220973$$
$$x_{47} = -5.87986312467225$$
$$x_{48} = 84.4295623053495$$
$$x_{49} = -61.6527420898353$$
$$x_{50} = 38.0901701050478$$
$$x_{51} = -45.9444322332416$$
$$x_{52} = 60.0819192595118$$
$$x_{53} = 42.0173142772847$$
$$x_{54} = -23.9520346967154$$
$$x_{55} = -64.0089738962685$$
$$x_{56} = -86.0003721530265$$
$$x_{57} = -17.6679214279049$$
$$x_{58} = 5.87986312467225$$
$$x_{59} = 96.2106254012507$$
$$x_{60} = -89.9273947056755$$
$$x_{61} = -53.7986124610881$$
$$x_{62} = -66.3652030529498$$
$$x_{63} = -49.8715301662158$$
$$x_{64} = 9.81110809029105$$
$$x_{65} = -39.6610314183159$$
$$x_{66} = 89.9273947056755$$
$$x_{67} = 34.1629906753197$$
$$x_{68} = 22.3810552326043$$
$$x_{69} = 20.0245321572363$$
$$x_{70} = -82.0733465634182$$
$$x_{71} = 30.2357622492879$$
$$x_{72} = 78.146317479006$$
$$x_{73} = 48.3006930832069$$
$$x_{74} = 52.2277811939441$$
$$x_{75} = -65.5797936108425$$
$$x_{76} = -57.725682309487$$
$$x_{77} = -75.0046913263314$$
$$x_{78} = 96.9960288247349$$
$$x_{79} = 26.3084628839094$$
$$x_{80} = 88.3565860231351$$
$$x_{81} = -30.2357622492879$$
$$x_{82} = -97.7814321637186$$
$$x_{83} = -31.8066606608336$$
$$x_{84} = 66.3652030529498$$
$$x_{85} = 100.13764169355$$
$$x_{86} = -1.93131295923443$$
$$x_{87} = 27.8793930615328$$
$$x_{88} = -21.5955555086822$$
$$x_{89} = -38.0901701050478$$
$$x_{90} = 86.0003721530265$$
$$x_{91} = 92.2836069408097$$
$$x_{92} = -93.8544146025812$$
$$x_{93} = -34.1629906753197$$
Signos de extremos en los puntos:
(-13.739919571972234, -0.0145537170574425)
(-43.58816420879817, -0.00458832607688831)
(40.44646018199142, -0.00494471404222622)
(71.86306224470708, -0.00278305409666654)
(44.3735877513282, 0.00450711416486993)
(70.29224648281348, -0.00284524602773579)
(18.453470150170162, -0.010837075476112)
(74.2192843450433, 0.00269470215409468)
(82.07334656341823, 0.00243683340006458)
(93.85441460258117, -0.00213095240453964)
(-12.168536057988597, -0.016432363233403)
(-60.08191925951185, 0.0033287596519433)
(394.66241874404795, 0.000506762109527025)
(-16.096779897639355, 0.0124233470459691)
(45.94443223324158, 0.00435302014867034)
(-42.0173142772847, -0.00475985824818994)
(23.952034696715426, 0.00834956650765266)
(-71.86306224470708, -0.00278305409666654)
(-27.879393061532753, -0.00717346891239463)
(12.168536057988597, -0.016432363233403)
(16.096779897639355, 0.0124233470459691)
(-9.811108090291048, 0.0203784424743046)
(75.00469132633138, -0.00266648506247399)
(-100.13764169354951, -0.00199724472573406)
(31.806660660833582, 0.00628779690129321)
(-83.6441571910727, 0.00239107085777116)
(-78.14631747900596, -0.00255928856302387)
(1.9313129592344267, 0.102699642820719)
(8.239097259955619, 0.0242633369490482)
(-59.296507317771535, -0.00337284996234339)
(-144.90552983176443, 0.00138020752265271)
(-79.7171295652025, -0.00250885873077445)
(-20.02453215723628, -0.00998697065742319)
(64.00897389626854, -0.00312453805134383)
(-56.15485569658162, -0.0035615445100986)
(56.15485569658162, -0.0035615445100986)
(62.438153004914184, -0.0032031440268943)
(67.9360211548046, 0.00294392639462058)
(53.79861246108815, 0.00371752785921583)
(-87.57118151564137, -0.00228384705497296)
(4.3051888179826925, -0.0463774418769231)
(49.871530166215834, -0.00401025367547625)
(-75.79009812486125, 0.00263885279068767)
(-67.9360211548046, 0.00294392639462058)
(-68.72142983243522, -0.00291028107370646)
(-35.73386742209732, -0.00559679430121812)
(-5.8798631246722515, -0.0339836935820901)
(84.4295623053495, -0.00236882814223086)
(-61.65274208983533, 0.00324394907611641)
(38.09017010504781, 0.00525058543453313)
(-45.94443223324158, 0.00435302014867034)
(60.08191925951185, 0.0033287596519433)
(42.0173142772847, -0.00475985824818994)
(-23.952034696715426, 0.00834956650765266)
(-64.00897389626854, -0.00312453805134383)
(-86.0003721530265, -0.00232556150577369)
(-17.667921427904876, 0.0113188176518931)
(5.8798631246722515, -0.0339836935820901)
(96.21062540125068, 0.00207876545825337)
(-89.92739470567548, 0.00222400779882111)
(-53.79861246108815, 0.00371752785921583)
(-66.36520305294982, 0.00301360610319761)
(-49.871530166215834, -0.00401025367547625)
(9.811108090291048, 0.0203784424743046)
(-39.66103141831586, 0.00504263302441695)
(89.92739470567548, 0.00222400779882111)
(34.16299067531975, -0.00585413165397496)
(22.381055232604297, 0.00893557170733885)
(20.02453215723628, -0.00998697065742319)
(-82.07334656341823, 0.00243683340006458)
(30.235762249287873, 0.00661445748028606)
(78.14631747900596, -0.00255928856302387)
(48.300693083206895, -0.00414067186791142)
(52.227781193944075, 0.00382933573271186)
(-65.5797936108425, -0.00304969771564102)
(-57.72568230948696, -0.00346462989119866)
(-75.00469132633138, -0.00266648506247399)
(96.99602882473488, -0.00206193323700578)
(26.3084628839094, -0.00760177329502683)
(88.35658602313505, 0.00226354602901002)
(-30.235762249287873, 0.00661445748028606)
(-97.78143216371859, 0.00204537141555956)
(-31.806660660833582, 0.00628779690129321)
(66.36520305294982, 0.00301360610319761)
(100.13764169354951, -0.00199724472573406)
(-1.9313129592344267, 0.102699642820719)
(27.879393061532753, -0.00717346891239463)
(-21.595555508682178, -0.00926054436677421)
(-38.09017010504781, 0.00525058543453313)
(86.0003721530265, -0.00232556150577369)
(92.28360694080966, -0.00216722419879452)
(-93.85441460258117, -0.00213095240453964)
(-34.16299067531975, -0.00585413165397496)
Intervalos de crecimiento y decrecimiento de la función:Hallemos los intervalos donde la función crece y decrece y también los puntos mínimos y máximos de la función, para lo cual miramos cómo se comporta la función en los extremos con desviación mínima del extremo:
Puntos mínimos de la función:
$$x_{1} = -13.7399195719722$$
$$x_{2} = -43.5881642087982$$
$$x_{3} = 40.4464601819914$$
$$x_{4} = 71.8630622447071$$
$$x_{5} = 70.2922464828135$$
$$x_{6} = 18.4534701501702$$
$$x_{7} = 93.8544146025812$$
$$x_{8} = -12.1685360579886$$
$$x_{9} = -42.0173142772847$$
$$x_{10} = -71.8630622447071$$
$$x_{11} = -27.8793930615328$$
$$x_{12} = 12.1685360579886$$
$$x_{13} = 75.0046913263314$$
$$x_{14} = -100.13764169355$$
$$x_{15} = -78.146317479006$$
$$x_{16} = -59.2965073177715$$
$$x_{17} = -79.7171295652025$$
$$x_{18} = -20.0245321572363$$
$$x_{19} = 64.0089738962685$$
$$x_{20} = -56.1548556965816$$
$$x_{21} = 56.1548556965816$$
$$x_{22} = 62.4381530049142$$
$$x_{23} = -87.5711815156414$$
$$x_{24} = 4.30518881798269$$
$$x_{25} = 49.8715301662158$$
$$x_{26} = -68.7214298324352$$
$$x_{27} = -35.7338674220973$$
$$x_{28} = -5.87986312467225$$
$$x_{29} = 84.4295623053495$$
$$x_{30} = 42.0173142772847$$
$$x_{31} = -64.0089738962685$$
$$x_{32} = -86.0003721530265$$
$$x_{33} = 5.87986312467225$$
$$x_{34} = -49.8715301662158$$
$$x_{35} = 34.1629906753197$$
$$x_{36} = 20.0245321572363$$
$$x_{37} = 78.146317479006$$
$$x_{38} = 48.3006930832069$$
$$x_{39} = -65.5797936108425$$
$$x_{40} = -57.725682309487$$
$$x_{41} = -75.0046913263314$$
$$x_{42} = 96.9960288247349$$
$$x_{43} = 26.3084628839094$$
$$x_{44} = 100.13764169355$$
$$x_{45} = 27.8793930615328$$
$$x_{46} = -21.5955555086822$$
$$x_{47} = 86.0003721530265$$
$$x_{48} = 92.2836069408097$$
$$x_{49} = -93.8544146025812$$
$$x_{50} = -34.1629906753197$$
Puntos máximos de la función:
$$x_{50} = 44.3735877513282$$
$$x_{50} = 74.2192843450433$$
$$x_{50} = 82.0733465634182$$
$$x_{50} = -60.0819192595118$$
$$x_{50} = 394.662418744048$$
$$x_{50} = -16.0967798976394$$
$$x_{50} = 45.9444322332416$$
$$x_{50} = 23.9520346967154$$
$$x_{50} = 16.0967798976394$$
$$x_{50} = -9.81110809029105$$
$$x_{50} = 31.8066606608336$$
$$x_{50} = -83.6441571910727$$
$$x_{50} = 1.93131295923443$$
$$x_{50} = 8.23909725995562$$
$$x_{50} = -144.905529831764$$
$$x_{50} = 67.9360211548046$$
$$x_{50} = 53.7986124610881$$
$$x_{50} = -75.7900981248612$$
$$x_{50} = -67.9360211548046$$
$$x_{50} = -61.6527420898353$$
$$x_{50} = 38.0901701050478$$
$$x_{50} = -45.9444322332416$$
$$x_{50} = 60.0819192595118$$
$$x_{50} = -23.9520346967154$$
$$x_{50} = -17.6679214279049$$
$$x_{50} = 96.2106254012507$$
$$x_{50} = -89.9273947056755$$
$$x_{50} = -53.7986124610881$$
$$x_{50} = -66.3652030529498$$
$$x_{50} = 9.81110809029105$$
$$x_{50} = -39.6610314183159$$
$$x_{50} = 89.9273947056755$$
$$x_{50} = 22.3810552326043$$
$$x_{50} = -82.0733465634182$$
$$x_{50} = 30.2357622492879$$
$$x_{50} = 52.2277811939441$$
$$x_{50} = 88.3565860231351$$
$$x_{50} = -30.2357622492879$$
$$x_{50} = -97.7814321637186$$
$$x_{50} = -31.8066606608336$$
$$x_{50} = 66.3652030529498$$
$$x_{50} = -1.93131295923443$$
$$x_{50} = -38.0901701050478$$
Decrece en los intervalos
$$\left[100.13764169355, \infty\right)$$
Crece en los intervalos
$$\left(-\infty, -100.13764169355\right]$$