El gráfico de la función cruce el eje X con f = 0
o sea hay que resolver la ecuación:
$$\left(- 9 x^{2} + \left(\frac{3 x^{4}}{4} - x^{3}\right)\right) + 7 = 0$$
Resolvermos esta ecuaciónPuntos de cruce con el eje X:
Solución analítica$$x_{1} = \frac{1}{3} - \frac{\sqrt{\frac{76}{9} + \frac{128}{9 \sqrt[3]{\frac{316}{27} + \frac{28 \sqrt{23} i}{9}}} + 2 \sqrt[3]{\frac{316}{27} + \frac{28 \sqrt{23} i}{9}}}}{2} - \frac{\sqrt{\frac{152}{9} - 2 \sqrt[3]{\frac{316}{27} + \frac{28 \sqrt{23} i}{9}} - \frac{448}{27 \sqrt{\frac{76}{9} + \frac{128}{9 \sqrt[3]{\frac{316}{27} + \frac{28 \sqrt{23} i}{9}}} + 2 \sqrt[3]{\frac{316}{27} + \frac{28 \sqrt{23} i}{9}}}} - \frac{128}{9 \sqrt[3]{\frac{316}{27} + \frac{28 \sqrt{23} i}{9}}}}}{2}$$
$$x_{2} = \frac{1}{3} + \frac{\sqrt{\frac{152}{9} - 2 \sqrt[3]{\frac{316}{27} + \frac{28 \sqrt{23} i}{9}} - \frac{448}{27 \sqrt{\frac{76}{9} + \frac{128}{9 \sqrt[3]{\frac{316}{27} + \frac{28 \sqrt{23} i}{9}}} + 2 \sqrt[3]{\frac{316}{27} + \frac{28 \sqrt{23} i}{9}}}} - \frac{128}{9 \sqrt[3]{\frac{316}{27} + \frac{28 \sqrt{23} i}{9}}}}}{2} - \frac{\sqrt{\frac{76}{9} + \frac{128}{9 \sqrt[3]{\frac{316}{27} + \frac{28 \sqrt{23} i}{9}}} + 2 \sqrt[3]{\frac{316}{27} + \frac{28 \sqrt{23} i}{9}}}}{2}$$
$$x_{3} = \frac{1}{3} - \frac{\sqrt{\frac{152}{9} - 2 \sqrt[3]{\frac{316}{27} + \frac{28 \sqrt{23} i}{9}} + \frac{448}{27 \sqrt{\frac{76}{9} + \frac{128}{9 \sqrt[3]{\frac{316}{27} + \frac{28 \sqrt{23} i}{9}}} + 2 \sqrt[3]{\frac{316}{27} + \frac{28 \sqrt{23} i}{9}}}} - \frac{128}{9 \sqrt[3]{\frac{316}{27} + \frac{28 \sqrt{23} i}{9}}}}}{2} + \frac{\sqrt{\frac{76}{9} + \frac{128}{9 \sqrt[3]{\frac{316}{27} + \frac{28 \sqrt{23} i}{9}}} + 2 \sqrt[3]{\frac{316}{27} + \frac{28 \sqrt{23} i}{9}}}}{2}$$
$$x_{4} = \frac{1}{3} + \frac{\sqrt{\frac{152}{9} - 2 \sqrt[3]{\frac{316}{27} + \frac{28 \sqrt{23} i}{9}} + \frac{448}{27 \sqrt{\frac{76}{9} + \frac{128}{9 \sqrt[3]{\frac{316}{27} + \frac{28 \sqrt{23} i}{9}}} + 2 \sqrt[3]{\frac{316}{27} + \frac{28 \sqrt{23} i}{9}}}} - \frac{128}{9 \sqrt[3]{\frac{316}{27} + \frac{28 \sqrt{23} i}{9}}}}}{2} + \frac{\sqrt{\frac{76}{9} + \frac{128}{9 \sqrt[3]{\frac{316}{27} + \frac{28 \sqrt{23} i}{9}}} + 2 \sqrt[3]{\frac{316}{27} + \frac{28 \sqrt{23} i}{9}}}}{2}$$
Solución numérica$$x_{1} = 4.11534047171244$$
$$x_{2} = -0.97910908564059$$
$$x_{3} = 0.867431309294424$$
$$x_{4} = -2.67032936203294$$