El gráfico de la función cruce el eje X con f = 0
o sea hay que resolver la ecuación:
$$\left(\sin^{2}{\left(x \right)} + 9 \cos^{2}{\left(x \right)}\right) + 6 \sin{\left(x \right)} \cos{\left(x \right)} = 0$$
Resolvermos esta ecuaciónPuntos de cruce con el eje X:
Solución analítica$$x_{1} = 2 \operatorname{atan}{\left(\frac{1}{3} - \frac{\sqrt{10}}{3} \right)}$$
$$x_{2} = 2 \operatorname{atan}{\left(\frac{1}{3} + \frac{\sqrt{10}}{3} \right)}$$
Solución numérica$$x_{1} = -45.231343149089$$
$$x_{2} = -57.7977134955084$$
$$x_{3} = 77.2907705646489$$
$$x_{4} = -60.9393062445047$$
$$x_{5} = 23.8836955465894$$
$$x_{6} = -32.6649720514865$$
$$x_{7} = 33.3084736739373$$
$$x_{8} = -98.6384177779503$$
$$x_{9} = -1.2490459945661$$
$$x_{10} = 99.2819193982116$$
$$x_{11} = 49.0164368010426$$
$$x_{12} = -48.3729353746117$$
$$x_{13} = -98.6384177580855$$
$$x_{14} = 89.8571412866408$$
$$x_{15} = -76.6472692023526$$
$$x_{16} = -54.6561206980047$$
$$x_{17} = 49.0164364389653$$
$$x_{18} = 14.4589173786987$$
$$x_{19} = 27.0252878650841$$
$$x_{20} = 11.3173250989179$$
$$x_{21} = 5.03413967456748$$
$$x_{22} = 99.2819191175832$$
$$x_{23} = 86.7155483153231$$
$$x_{24} = 92.9987339229954$$
$$x_{25} = -82.9304545889042$$
$$x_{26} = 71.0075850130346$$
$$x_{27} = 96.1403266418809$$
$$x_{28} = 64.7243997377488$$
$$x_{29} = -86.0720473565394$$
$$x_{30} = -79.788862079548$$
$$x_{31} = -35.8065649106107$$
$$x_{32} = -64.0808987755242$$
$$x_{33} = 20.7421025828029$$
$$x_{34} = 52.1580294444207$$
$$x_{35} = -60.9393060219151$$
$$x_{36} = -67.2224917262399$$
$$x_{37} = 42.7332516210452$$
$$x_{38} = 55.2996220139031$$
$$x_{39} = -76.6472692300561$$
$$x_{40} = 33.3084734655833$$
$$x_{41} = 23.8836954922314$$
$$x_{42} = -32.6649721621368$$
$$x_{43} = 83.5739560922884$$
$$x_{44} = 80.4323631165255$$
$$x_{45} = -38.9481574556275$$
$$x_{46} = 42.7332511602417$$
$$x_{47} = 17.6005103591293$$
$$x_{48} = -51.5145283586584$$
$$x_{49} = -92.3552329783259$$
$$x_{50} = 67.8659927067436$$
$$x_{51} = -10.6738234762247$$
$$x_{52} = 36.4500659579211$$
$$x_{53} = 27.0252882383159$$
$$x_{54} = -38.9481578553278$$
$$x_{55} = -13.8154163246086$$
$$x_{56} = -20.0986016141231$$
$$x_{57} = -89.2136403033148$$
$$x_{58} = 45.8748441267298$$
$$x_{59} = -54.6561206268636$$
$$x_{60} = 74.1491780404601$$
$$x_{61} = 61.5828075146345$$
$$x_{62} = -4.39063821872051$$
$$x_{63} = 5.03413929140085$$
$$x_{64} = -29.5233797795905$$
$$x_{65} = 30.1668808522687$$
$$x_{66} = -73.5056769376756$$
$$x_{67} = -92.3552325307549$$
$$x_{68} = -42.0897501947274$$
$$x_{69} = 55.2996222488292$$
$$x_{70} = -82.9304550011229$$
$$x_{71} = 92.998733587283$$
$$x_{72} = -16.9570088901083$$
$$x_{73} = 1.89254696630993$$
$$x_{74} = 64.7243999545554$$
$$x_{75} = -95.4968255166424$$
$$x_{76} = 58.4412145371964$$
$$x_{77} = -23.2401945718639$$
$$x_{78} = 8.17573226284759$$
$$x_{79} = -26.3817867966344$$
$$x_{80} = -60.9393064283489$$
$$x_{81} = -10.6738236227216$$
$$x_{82} = -70.3640839526521$$
$$x_{83} = 11.3173249199489$$
$$x_{84} = 39.5916589369146$$
$$x_{85} = -16.9570092820737$$
$$x_{86} = 77.290770823589$$
$$x_{87} = 71.0075853626419$$
$$x_{88} = -7.53223120047166$$