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$$15 x^{4} \cos{\left(3 x \right)} + 20 x^{3} \sin{\left(3 x \right)} = 0$$
Resolvermos esta ecuaciónRaíces de esta ecuación
$$x_{1} = -91.634635567103$$
$$x_{2} = -63.3624651297861$$
$$x_{3} = 44.5158768953691$$
$$x_{4} = 40.3281224156889$$
$$x_{5} = -60.221238182616$$
$$x_{6} = 73.8334462595067$$
$$x_{7} = 12.079417026695$$
$$x_{8} = 49.7508149542794$$
$$x_{9} = -36.1406075887119$$
$$x_{10} = 75.9276753838356$$
$$x_{11} = 64.4095487013249$$
$$x_{12} = 53.9389119829474$$
$$x_{13} = -1.78467728039067$$
$$x_{14} = -2.76764306086567$$
$$x_{15} = 3.77827519433957$$
$$x_{16} = -49.7508149542794$$
$$x_{17} = 93.7289223191141$$
$$x_{18} = -75.9276753838356$$
$$x_{19} = -65.4566359175226$$
$$x_{20} = -21.4882064963395$$
$$x_{21} = 88.4932150522796$$
$$x_{22} = 31.9534263797684$$
$$x_{23} = 22.5344472736599$$
$$x_{24} = -82.2104134768321$$
$$x_{25} = 51.8448494812653$$
$$x_{26} = 14.1684434456615$$
$$x_{27} = -67.550820606457$$
$$x_{28} = -47.6568120846336$$
$$x_{29} = 80.1161596550644$$
$$x_{30} = -78.0219134150581$$
$$x_{31} = 100.011809795649$$
$$x_{32} = 18.3501348839456$$
$$x_{33} = 20.4420631577666$$
$$x_{34} = 86.3989416613461$$
$$x_{35} = -18.3501348839456$$
$$x_{36} = 66.5037266063558$$
$$x_{37} = 0$$
$$x_{38} = 5.83447607252139$$
$$x_{39} = 78.0219134150581$$
$$x_{40} = 7.9096484587492$$
$$x_{41} = 29.8600046009359$$
$$x_{42} = -31.9534263797684$$
$$x_{43} = -12.079417026695$$
$$x_{44} = 95.8232138061131$$
$$x_{45} = -9.99259294922328$$
$$x_{46} = 90.5874940693044$$
$$x_{47} = 82.2104134768321$$
$$x_{48} = -51.8448494812653$$
$$x_{49} = 71.7392268216346$$
$$x_{50} = -5.83447607252139$$
$$x_{51} = 43.4689195888571$$
$$x_{52} = -71.7392268216346$$
$$x_{53} = -25.6736357826784$$
$$x_{54} = 36.1406075887119$$
$$x_{55} = -38.234330134717$$
$$x_{56} = -23.5807726014381$$
$$x_{57} = 38.234330134717$$
$$x_{58} = -27.7667291875486$$
$$x_{59} = -73.8334462595067$$
$$x_{60} = -14.1684434456615$$
$$x_{61} = -53.9389119829474$$
$$x_{62} = -13.1237193239869$$
$$x_{63} = -93.7289223191141$$
$$x_{64} = -87.4460776282981$$
$$x_{65} = -69.6450179434699$$
$$x_{66} = -43.4689195888571$$
$$x_{67} = -56.0329993263719$$
$$x_{68} = 26.7201570725466$$
$$x_{69} = 97.9175097243864$$
$$x_{70} = -97.9175097243864$$
$$x_{71} = -7.9096484587492$$
$$x_{72} = -95.8232138061131$$
$$x_{73} = 84.3046743156037$$
$$x_{74} = 34.0469676095731$$
$$x_{75} = -100.011809795649$$
$$x_{76} = 58.1271088294339$$
$$x_{77} = -58.1271088294339$$
$$x_{78} = -84.3046743156037$$
$$x_{79} = -80.1161596550644$$
$$x_{80} = -3.77827519433957$$
$$x_{81} = -29.8600046009359$$
$$x_{82} = 56.0329993263719$$
$$x_{83} = -35.0937763570717$$
$$x_{84} = -16.2588365740921$$
$$x_{85} = -50.7978285049785$$
$$x_{86} = -34.0469676095731$$
$$x_{87} = 62.315385386479$$
$$x_{88} = 60.221238182616$$
$$x_{89} = 9.99259294922328$$
$$x_{90} = -45.5628452306873$$
$$x_{91} = 42.4219741262856$$
$$x_{92} = -89.5403538821935$$
$$x_{93} = 16.2588365740921$$
$$x_{94} = 27.7667291875486$$
$$x_{95} = 1.78467728039067$$
$$x_{96} = 21.4882064963395$$
Signos de extremos en los puntos:
(-91.63463556710299, 352502882.774754)
(-63.36246512978607, -80575329.3464298)
(44.5158768953691, 19626153.617958)
(40.32812241568886, 13217978.5642116)
(-60.22123818261599, 65744943.8717191)
(73.83344625950674, 148563384.43735)
(12.079417026694966, -105801.386583899)
(49.75081495427937, -30620676.205518)
(-36.14060758871191, -8524247.27036998)
(75.9276753838356, 166151202.06203)
(64.40954870132492, -86035553.1893984)
(53.93891198294738, -42310290.1652217)
(-1.7846772803906716, 48.6352438938571)
(-2.7676430608656735, -256.295044714754)
(3.7782751943395687, -952.855158790833)
(-49.75081495427937, 30620692.205518)
(93.72892231911408, -385851818.778314)
(-75.9276753838356, -166151186.06203)
(-65.45663591752259, -91768706.6093194)
(-21.4882064963395, -1063978.7911248)
(88.49321505227961, 306591860.100884)
(31.953426379768374, 5207900.12662698)
(22.53444727365992, -1287051.88610351)
(-82.21041347683207, -228360091.27821)
(51.84484948126533, -36111772.4828875)
(14.168443445661534, -200598.036368131)
(-67.550820606457, -104089735.311377)
(-47.65681208463359, 25781025.1444908)
(80.11615965506437, 205963554.862822)
(-78.02191341505808, -185256291.490013)
(100.01180979564857, -500191780.740036)
(18.350134883945593, -565428.212317855)
(20.44206315776662, -871250.559055239)
(86.39894166134607, 278581324.408907)
(-18.350134883945593, 565444.212317855)
(66.50372660635584, -97783736.8115913)
(0, 8)
(5.8344760725213884, -5640.3716615012)
(78.02191341505808, 185256307.490013)
(7.909648458749203, -19290.0895429007)
(29.860004600935945, 3970981.16650358)
(-31.953426379768374, -5207884.12662698)
(-12.079417026694966, 105817.386583899)
(95.82321380611309, -421512916.768012)
(-9.99259294922328, 49422.0765649696)
(90.58749406930437, 336663442.788779)
(82.21041347683207, 228360107.27821)
(-51.84484948126533, 36111788.4828875)
(71.73922682163465, 132410309.430682)
(-5.8344760725213884, 5656.3716615012)
(43.46891958885709, -17843538.6981349)
(-71.73922682163465, -132410293.430682)
(-25.673635782678396, -2169366.94774642)
(36.14060758871191, 8524263.27036998)
(-38.234330134716956, -10678732.5315033)
(-23.580772601438067, -1543500.34903306)
(38.234330134716956, 10678748.5315033)
(-27.766729187548563, -2968708.82293071)
(-73.83344625950674, -148563368.43735)
(-14.168443445661534, 200614.036368131)
(-53.93891198294738, 42310306.1652217)
(-13.12371932398686, -147551.724253833)
(-93.72892231911408, 385851834.778314)
(-87.44607762829814, 292335040.429235)
(-69.64501794346985, -117611724.812966)
(-43.46891958885709, 17843554.6981349)
(-56.03299932637194, 49274546.3973751)
(26.720157072546627, -2545567.64244689)
(97.9175097243864, -459590641.013344)
(-97.9175097243864, 459590657.013344)
(-7.909648458749203, 19306.0895429007)
(-95.82321380611309, 421512932.768012)
(84.30467431560369, 252535436.790166)
(34.046967609573116, 6713538.80770276)
(-100.01180979564857, 500191796.740036)
(58.1271088294339, -57065103.2804346)
(-58.1271088294339, 57065119.2804346)
(-84.30467431560369, -252535420.790166)
(-80.11615965506437, -205963538.862822)
(-3.7782751943395687, 968.855158790833)
(-29.860004600935945, -3970965.16650358)
(56.03299932637194, -49274530.3973751)
(-35.093776357071675, 7578402.24394453)
(-16.258836574092104, 348243.000943114)
(-50.79782850497848, -33281350.2965328)
(-34.046967609573116, -6713522.80770276)
(62.31538538647896, -75379232.1847793)
(60.22123818261599, -65744927.8717191)
(9.99259294922328, -49406.0765649696)
(-45.56284523068725, 21539104.0674156)
(42.421974126285555, 16185246.2978379)
(-89.54035388219354, 321363905.444071)
(16.258836574092104, -348227.000943114)
(27.766729187548563, 2968724.82293071)
(1.7846772803906716, -32.6352438938571)
(21.4882064963395, 1063994.7911248)
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:
$$x_{1} = -63.3624651297861$$
$$x_{2} = 12.079417026695$$
$$x_{3} = 49.7508149542794$$
$$x_{4} = -36.1406075887119$$
$$x_{5} = 64.4095487013249$$
$$x_{6} = 53.9389119829474$$
$$x_{7} = -2.76764306086567$$
$$x_{8} = 3.77827519433957$$
$$x_{9} = 93.7289223191141$$
$$x_{10} = -75.9276753838356$$
$$x_{11} = -65.4566359175226$$
$$x_{12} = -21.4882064963395$$
$$x_{13} = 22.5344472736599$$
$$x_{14} = -82.2104134768321$$
$$x_{15} = 51.8448494812653$$
$$x_{16} = 14.1684434456615$$
$$x_{17} = -67.550820606457$$
$$x_{18} = -78.0219134150581$$
$$x_{19} = 100.011809795649$$
$$x_{20} = 18.3501348839456$$
$$x_{21} = 20.4420631577666$$
$$x_{22} = 66.5037266063558$$
$$x_{23} = 5.83447607252139$$
$$x_{24} = 7.9096484587492$$
$$x_{25} = -31.9534263797684$$
$$x_{26} = 95.8232138061131$$
$$x_{27} = 43.4689195888571$$
$$x_{28} = -71.7392268216346$$
$$x_{29} = -25.6736357826784$$
$$x_{30} = -38.234330134717$$
$$x_{31} = -23.5807726014381$$
$$x_{32} = -27.7667291875486$$
$$x_{33} = -73.8334462595067$$
$$x_{34} = -13.1237193239869$$
$$x_{35} = -69.6450179434699$$
$$x_{36} = 26.7201570725466$$
$$x_{37} = 97.9175097243864$$
$$x_{38} = 58.1271088294339$$
$$x_{39} = -84.3046743156037$$
$$x_{40} = -80.1161596550644$$
$$x_{41} = -29.8600046009359$$
$$x_{42} = 56.0329993263719$$
$$x_{43} = -50.7978285049785$$
$$x_{44} = -34.0469676095731$$
$$x_{45} = 62.315385386479$$
$$x_{46} = 60.221238182616$$
$$x_{47} = 9.99259294922328$$
$$x_{48} = 16.2588365740921$$
$$x_{49} = 1.78467728039067$$
Puntos máximos de la función:
$$x_{49} = -91.634635567103$$
$$x_{49} = 44.5158768953691$$
$$x_{49} = 40.3281224156889$$
$$x_{49} = -60.221238182616$$
$$x_{49} = 73.8334462595067$$
$$x_{49} = 75.9276753838356$$
$$x_{49} = -1.78467728039067$$
$$x_{49} = -49.7508149542794$$
$$x_{49} = 88.4932150522796$$
$$x_{49} = 31.9534263797684$$
$$x_{49} = -47.6568120846336$$
$$x_{49} = 80.1161596550644$$
$$x_{49} = 86.3989416613461$$
$$x_{49} = -18.3501348839456$$
$$x_{49} = 78.0219134150581$$
$$x_{49} = 29.8600046009359$$
$$x_{49} = -12.079417026695$$
$$x_{49} = -9.99259294922328$$
$$x_{49} = 90.5874940693044$$
$$x_{49} = 82.2104134768321$$
$$x_{49} = -51.8448494812653$$
$$x_{49} = 71.7392268216346$$
$$x_{49} = -5.83447607252139$$
$$x_{49} = 36.1406075887119$$
$$x_{49} = 38.234330134717$$
$$x_{49} = -14.1684434456615$$
$$x_{49} = -53.9389119829474$$
$$x_{49} = -93.7289223191141$$
$$x_{49} = -87.4460776282981$$
$$x_{49} = -43.4689195888571$$
$$x_{49} = -56.0329993263719$$
$$x_{49} = -97.9175097243864$$
$$x_{49} = -7.9096484587492$$
$$x_{49} = -95.8232138061131$$
$$x_{49} = 84.3046743156037$$
$$x_{49} = 34.0469676095731$$
$$x_{49} = -100.011809795649$$
$$x_{49} = -58.1271088294339$$
$$x_{49} = -3.77827519433957$$
$$x_{49} = -35.0937763570717$$
$$x_{49} = -16.2588365740921$$
$$x_{49} = -45.5628452306873$$
$$x_{49} = 42.4219741262856$$
$$x_{49} = -89.5403538821935$$
$$x_{49} = 27.7667291875486$$
$$x_{49} = 21.4882064963395$$
Decrece en los intervalos
$$\left[100.011809795649, \infty\right)$$
Crece en los intervalos
$$\left(-\infty, -84.3046743156037\right]$$