Graph of the function intersects the axis X at f = 0
so we need to solve the equation:
$$\cos^{2}{\left(3 x \right)} = 0$$
Solve this equationThe points of intersection with the axis X:
Analytical solution$$x_{1} = \frac{\pi}{6}$$
$$x_{2} = \frac{\pi}{2}$$
Numerical solution$$x_{1} = -36.1283154422719$$
$$x_{2} = 42.4115007524084$$
$$x_{3} = -47.6474886360566$$
$$x_{4} = 75.9218225202323$$
$$x_{5} = -34.0339203815254$$
$$x_{6} = -43.4586982782211$$
$$x_{7} = 0.523598793698386$$
$$x_{8} = 219.387886624637$$
$$x_{9} = -67.5442421196901$$
$$x_{10} = -31.9395254607427$$
$$x_{11} = -3.6651914786486$$
$$x_{12} = 51.8362788611105$$
$$x_{13} = 100.007366129875$$
$$x_{14} = -95.8185758690479$$
$$x_{15} = -9.94837684644357$$
$$x_{16} = 78.0162175417499$$
$$x_{17} = -51.8362786947358$$
$$x_{18} = 84.2994028394378$$
$$x_{19} = -21.467549748169$$
$$x_{20} = -93.7241808498957$$
$$x_{21} = 18.3259571004719$$
$$x_{22} = -65.4498467952353$$
$$x_{23} = -49.7418836877244$$
$$x_{24} = 27.7507351416945$$
$$x_{25} = -60.2138591280511$$
$$x_{26} = -12.0427718000216$$
$$x_{27} = -62.3082544993577$$
$$x_{28} = 34.0339203573462$$
$$x_{29} = 53.9306739409302$$
$$x_{30} = 73.8274274357866$$
$$x_{31} = -1.57079639846571$$
$$x_{32} = 20.4203521766952$$
$$x_{33} = -7.85398152418181$$
$$x_{34} = 93.7241808065961$$
$$x_{35} = 82.205007807797$$
$$x_{36} = -89.5353906915371$$
$$x_{37} = 38.2227106407841$$
$$x_{38} = 7.85398171030129$$
$$x_{39} = -56.0250689637531$$
$$x_{40} = 38.2227104061931$$
$$x_{41} = 62.308254259517$$
$$x_{42} = 5.75958657756439$$
$$x_{43} = 29.8451302859375$$
$$x_{44} = -5.75958652424928$$
$$x_{45} = 97.9129710990821$$
$$x_{46} = 12.0427717588149$$
$$x_{47} = 95.8185760099185$$
$$x_{48} = -25.6563400574942$$
$$x_{49} = -16.2315619891784$$
$$x_{50} = -71.7330322689618$$
$$x_{51} = -40.3171057809134$$
$$x_{52} = -96.8657733416992$$
$$x_{53} = 60.2138592240082$$
$$x_{54} = -82.2050076993983$$
$$x_{55} = 31.9395253611477$$
$$x_{56} = -87.4409953189147$$
$$x_{57} = -80.1106125965004$$
$$x_{58} = 44.5058959013899$$
$$x_{59} = -29.8451301070482$$
$$x_{60} = -23.5619449729929$$
$$x_{61} = -100.007366130902$$
$$x_{62} = 71.7330322578684$$
$$x_{63} = 22.51474734437$$
$$x_{64} = 66.4970444630068$$
$$x_{65} = 64.4026493286099$$
$$x_{66} = -58.1194640191823$$
$$x_{67} = -84.2994030191035$$
$$x_{68} = -69.6386372143248$$
$$x_{69} = -27.7507351061626$$
$$x_{70} = -14.1371668657739$$
$$x_{71} = -38.2227105578705$$
$$x_{72} = -91.6297857922845$$
$$x_{73} = 16.2315620580511$$
$$x_{74} = -53.9306740746488$$
$$x_{75} = 88.4881930280058$$
$$x_{76} = 86.3937979052498$$
$$x_{77} = -218.340689710019$$
$$x_{78} = 9.94837678084721$$
$$x_{79} = -96.865773562311$$
$$x_{80} = 49.7418837022546$$
$$x_{81} = 14.1371669873783$$
$$x_{82} = 56.0250689512033$$
$$x_{83} = 4.71238901629785$$
$$x_{84} = -78.0162175468244$$
$$x_{85} = -73.8274272823298$$
$$x_{86} = -45.5530935467806$$
$$x_{87} = 40.3171056798614$$