Graph of the function intersects the axis X at f = 0
so we need to solve the equation:
$$\cos{\left(\frac{x}{2} - \frac{2 \pi}{3} \right)} = 0$$
Solve this equationThe points of intersection with the axis X:
Analytical solution$$x_{1} = \frac{\pi}{3}$$
$$x_{2} = \frac{7 \pi}{3}$$
Numerical solution$$x_{1} = -55.5014702134197$$
$$x_{2} = -11.5191730631626$$
$$x_{3} = -42.9350995990605$$
$$x_{4} = -30.3687289847013$$
$$x_{5} = -33714.5251607745$$
$$x_{6} = -49.2182849062401$$
$$x_{7} = 38.7463093942741$$
$$x_{8} = -74.3510261349584$$
$$x_{9} = 51.3126800086333$$
$$x_{10} = 1.0471975511966$$
$$x_{11} = 95.2949771588904$$
$$x_{12} = -137.182879206754$$
$$x_{13} = -99.4837673636768$$
$$x_{14} = 26.1799387799149$$
$$x_{15} = 76.4454212373516$$
$$x_{16} = 57.5958653158129$$
$$x_{17} = 82.7286065445312$$
$$x_{18} = -507.89081233035$$
$$x_{19} = -61.7846555205993$$
$$x_{20} = -17.8023583703422$$
$$x_{21} = 45.0294947014537$$
$$x_{22} = 32.4631240870945$$
$$x_{23} = 70.162235930172$$
$$x_{24} = -80.634211442138$$
$$x_{25} = -17164.6150616634$$
$$x_{26} = 101.57816246607$$
$$x_{27} = -5.23598775598299$$
$$x_{28} = -68.0678408277789$$
$$x_{29} = 89.0117918517108$$
$$x_{30} = -24.0855436775217$$
$$x_{31} = -93.2005820564972$$
$$x_{32} = 63.8790506229925$$
$$x_{33} = 7.33038285837618$$
$$x_{34} = -36.6519142918809$$
$$x_{35} = 19.8967534727354$$
$$x_{36} = -86.9173967493176$$
$$x_{37} = 13.6135681655558$$