Graph of the function intersects the axis X at f = 0
so we need to solve the equation:
$$\left|{\cos{\left(3 x \right)}}\right| = 0$$
Solve this equationThe points of intersection with the axis X:
Analytical solution$$x_{1} = \frac{\pi}{6}$$
$$x_{2} = \frac{\pi}{2}$$
Numerical solution$$x_{1} = -8.90117918517108$$
$$x_{2} = -29.845130209103$$
$$x_{3} = -36.1283155162826$$
$$x_{4} = 84.2994028713261$$
$$x_{5} = 9.94837673636768$$
$$x_{6} = -39.2699081698724$$
$$x_{7} = 62.3082542961976$$
$$x_{8} = -5.75958653158129$$
$$x_{9} = 100.007366139275$$
$$x_{10} = -78.0162175641465$$
$$x_{11} = 75.9218224617533$$
$$x_{12} = -18.3259571459405$$
$$x_{13} = 15.1843644923507$$
$$x_{14} = 38.2227106186758$$
$$x_{15} = -100.007366139275$$
$$x_{16} = 12.0427718387609$$
$$x_{17} = -27.7507351067098$$
$$x_{18} = -34.0339204138894$$
$$x_{19} = 91.6297857297023$$
$$x_{20} = -1.5707963267949$$
$$x_{21} = 73.8274273593601$$
$$x_{22} = 51.8362787842316$$
$$x_{23} = -58.1194640914112$$
$$x_{24} = -3.66519142918809$$
$$x_{25} = -91.6297857297023$$
$$x_{26} = -80.1106126665397$$
$$x_{27} = 18.3259571459405$$
$$x_{28} = 78.0162175641465$$
$$x_{29} = 27.7507351067098$$
$$x_{30} = -71.733032256967$$
$$x_{31} = 40.317105721069$$
$$x_{32} = -51.8362787842316$$
$$x_{33} = -12.0427718387609$$
$$x_{34} = 7.85398163397448$$
$$x_{35} = -25.6563400043166$$
$$x_{36} = -45.553093477052$$
$$x_{37} = 86.3937979737193$$
$$x_{38} = -73.8274273593601$$
$$x_{39} = 97.9129710368819$$
$$x_{40} = 2.61799387799149$$
$$x_{41} = -47.6474885794452$$
$$x_{42} = -95.8185759344887$$
$$x_{43} = 56.025068989018$$
$$x_{44} = 20.4203522483337$$
$$x_{45} = -56.025068989018$$
$$x_{46} = 64.4026493985908$$
$$x_{47} = 34.0339204138894$$
$$x_{48} = 31.9395253114962$$
$$x_{49} = 53.9306738866248$$
$$x_{50} = 42.4115008234622$$
$$x_{51} = 16.2315620435473$$
$$x_{52} = -947782.375272524$$
$$x_{53} = -14.1371669411541$$
$$x_{54} = 82.2050077689329$$
$$x_{55} = -31.9395253114962$$
$$x_{56} = 95.8185759344887$$
$$x_{57} = -93.7241808320955$$
$$x_{58} = 54.9778714378214$$
$$x_{59} = -49.7418836818384$$
$$x_{60} = -69.6386371545737$$
$$x_{61} = 60.2138591938044$$
$$x_{62} = 29.845130209103$$