El gráfico de la función cruce el eje X con f = 0
o sea hay que resolver la ecuación:
$$\left(\left(\left(-4\right) \cos{\left(x \right)} - \frac{4 \cos{\left(3 x \right)}}{9}\right) + \frac{\pi^{2}}{3}\right) + \cos{\left(2 x \right)} = 0$$
Resolvermos esta ecuaciónPuntos de cruce con el eje X:
Solución numérica$$x_{1} = -68.8314391616408$$
$$x_{2} = -88.2481935178489$$
$$x_{3} = 69.3986375963101$$
$$x_{4} = -37.9827110604122$$
$$x_{5} = 5.99958608984492$$
$$x_{6} = -62.5482538544612$$
$$x_{7} = 63.1154522891305$$
$$x_{8} = -31.6995257532326$$
$$x_{9} = -81.397809776$$
$$x_{10} = -37.4155126257429$$
$$x_{11} = -44.2658963675918$$
$$x_{12} = -25.416340446053$$
$$x_{13} = 6.56678452451425$$
$$x_{14} = 81.9650082106693$$
$$x_{15} = -18.5659567042041$$
$$x_{16} = -75.1146244688204$$
$$x_{17} = -12.8499698316938$$
$$x_{18} = -63.1154522891305$$
$$x_{19} = 56.8322669819509$$
$$x_{20} = -49.981883240102$$
$$x_{21} = 88.2481935178489$$
$$x_{22} = 62.5482538544612$$
$$x_{23} = 31.1323273185633$$
$$x_{24} = -31.1323273185633$$
$$x_{25} = 43.6986979329224$$
$$x_{26} = -43.6986979329224$$
$$x_{27} = 94.5313788250285$$
$$x_{28} = 37.4155126257429$$
$$x_{29} = 12.8499698316938$$
$$x_{30} = -5.99958608984492$$
$$x_{31} = 24.8491420113837$$
$$x_{32} = -75.6818229034897$$
$$x_{33} = 8388.33598430208$$
$$x_{34} = -100.247365697539$$
$$x_{35} = -56.8322669819509$$
$$x_{36} = -6.56678452451425$$
$$x_{37} = -94.5313788250285$$
$$x_{38} = 81.397809776$$
$$x_{39} = -93.9641803903591$$
$$x_{40} = 50.5490816747714$$
$$x_{41} = 0.283599217334663$$
$$x_{42} = 100.247365697539$$
$$x_{43} = 93.9641803903591$$
$$x_{44} = 56.2650685472816$$
$$x_{45} = -50.5490816747714$$
$$x_{46} = 12.2827713970245$$
$$x_{47} = 44.2658963675918$$
$$x_{48} = 68.8314391616408$$
$$x_{49} = -81.9650082106693$$
$$x_{50} = -12.2827713970245$$
$$x_{51} = 87.6809950831796$$
$$x_{52} = -69.3986375963101$$
$$x_{53} = 49.981883240102$$
$$x_{54} = 25.416340446053$$
$$x_{55} = 37.9827110604122$$
$$x_{56} = 75.6818229034897$$
$$x_{57} = -24.8491420113837$$
$$x_{58} = -87.6809950831796$$
$$x_{59} = -112.813736311898$$
$$x_{60} = -19.1331551388734$$
$$x_{61} = 18.5659567042041$$
$$x_{62} = 19.1331551388734$$
$$x_{63} = -0.283599217334663$$
$$x_{64} = 31.6995257532326$$
$$x_{65} = -56.2650685472816$$
$$x_{66} = 75.1146244688204$$