Graph of the function intersects the axis X at f = 0
so we need to solve the equation:
$$\cos{\left(7 x \right)} = 0$$
Solve this equationThe points of intersection with the axis X:
Analytical solution$$x_{1} = \frac{\pi}{14}$$
$$x_{2} = \frac{3 \pi}{14}$$
Numerical solution$$x_{1} = 66.1978452006421$$
$$x_{2} = 14.1371669411541$$
$$x_{3} = 52.2850777347444$$
$$x_{4} = -32.53792391218$$
$$x_{5} = -7.85398163397448$$
$$x_{6} = -41.9627018729494$$
$$x_{7} = 56.7730672398727$$
$$x_{8} = -46.0018924275648$$
$$x_{9} = -19.9715532978208$$
$$x_{10} = 78.3154168644884$$
$$x_{11} = 100.306565439617$$
$$x_{12} = 6.05878583192317$$
$$x_{13} = 30.2939291596159$$
$$x_{14} = -87.7401948252578$$
$$x_{15} = -21.7667490998721$$
$$x_{16} = 80.1106126665397$$
$$x_{17} = 88.1889937757706$$
$$x_{18} = 44.2066966255135$$
$$x_{19} = -29.845130209103$$
$$x_{20} = -2.01959527730772$$
$$x_{21} = -89.9841895778219$$
$$x_{22} = -37.9235113183339$$
$$x_{23} = -69.7882368047447$$
$$x_{24} = 32.0891249616672$$
$$x_{25} = -43.7578976750007$$
$$x_{26} = -81.905808468591$$
$$x_{27} = -35.6795165657698$$
$$x_{28} = 63.9538504480779$$
$$x_{29} = -6.05878583192317$$
$$x_{30} = 37.4747123678211$$
$$x_{31} = -39.7187071203852$$
$$x_{32} = -98.0625706870528$$
$$x_{33} = -65.7490462501292$$
$$x_{34} = -50.0410829821803$$
$$x_{35} = 92.2281843303861$$
$$x_{36} = 28.0499344070517$$
$$x_{37} = 74.276226309873$$
$$x_{38} = 58.1194640914112$$
$$x_{39} = -72.0322315573088$$
$$x_{40} = -17.7275585452567$$
$$x_{41} = 94.0233801324374$$
$$x_{42} = -59.9146598934625$$
$$x_{43} = 50.0410829821803$$
$$x_{44} = -51.8362787842316$$
$$x_{45} = -73.8274273593601$$
$$x_{46} = -67.9930410026934$$
$$x_{47} = -76.0714221119243$$
$$x_{48} = 15.9323627432054$$
$$x_{49} = -95.8185759344887$$
$$x_{50} = 70.2370357552575$$
$$x_{51} = -63.9538504480779$$
$$x_{52} = 18.1763574957695$$
$$x_{53} = 72.0322315573088$$
$$x_{54} = -28.0499344070517$$
$$x_{55} = 41.9627018729494$$
$$x_{56} = -55.875469338847$$
$$x_{57} = 0.224399475256414$$
$$x_{58} = -47.7970882296161$$
$$x_{59} = -24.0107438524363$$
$$x_{60} = 62.1586546460266$$
$$x_{61} = 10.0979763865386$$
$$x_{62} = 84.1498032211552$$
$$x_{63} = -25.8059396544876$$
$$x_{64} = 96.2673748850015$$
$$x_{65} = -85.9449990232065$$
$$x_{66} = 3.81479107935903$$
$$x_{67} = -91.7793853798732$$
$$x_{68} = 26.2547386050004$$
$$x_{69} = 19.9715532978208$$
$$x_{70} = 85.9449990232065$$
$$x_{71} = -11.8931721885899$$
$$x_{72} = 48.245887180129$$
$$x_{73} = -99.8577664891041$$
$$x_{74} = 54.0802735367957$$
$$x_{75} = -77.8666179139756$$
$$x_{76} = 8.30278058448731$$
$$x_{77} = -3.81479107935903$$
$$x_{78} = -33.8843207637185$$
$$x_{79} = 46.0018924275648$$
$$x_{80} = -61.7098556955138$$
$$x_{81} = 36.1283155162826$$
$$x_{82} = 76.0714221119243$$
$$x_{83} = 22.215548050385$$
$$x_{84} = -80.5594116170526$$
$$x_{85} = 4.26359002987186$$
$$x_{86} = -83.7010042706423$$
$$x_{87} = 40.1675060708981$$
$$x_{88} = -13.6883679906412$$
$$x_{89} = 2.01959527730772$$
$$x_{90} = -15.9323627432054$$
$$x_{91} = 98.0625706870528$$
$$x_{92} = 89.9841895778219$$
$$x_{93} = -94.0233801324374$$
$$x_{94} = 24.0107438524363$$
$$x_{95} = 67.9930410026934$$