Sr Examen
ab+ac+bc forma canónica
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Solución
Método de invariantes
Se da la ecuación de superficie de 2 grado:
$$a b + a c + b c = 0$$
Esta ecuación tiene la forma:
$$a^{2} a_{33} + 2 a a_{13} c + 2 a a_{23} b + 2 a a_{34} + a_{11} c^{2} + 2 a_{12} b c + 2 a_{14} c + a_{22} b^{2} + 2 a_{24} b + a_{44} = 0$$
donde
$$a_{11} = 0$$
$$a_{12} = \frac{1}{2}$$
$$a_{13} = \frac{1}{2}$$
$$a_{14} = 0$$
$$a_{22} = 0$$
$$a_{23} = \frac{1}{2}$$
$$a_{24} = 0$$
$$a_{33} = 0$$
$$a_{34} = 0$$
$$a_{44} = 0$$
Las invariantes de esta ecuación al transformar las coordenadas son los determinantes:
$$I_{1} = a_{11} + a_{22} + a_{33}$$
$$I_{3} = \left|\begin{matrix}a_{11} & a_{12} & a_{13}\\a_{12} & a_{22} & a_{23}\\a_{13} & a_{23} & a_{33}\end{matrix}\right|$$
$$I_{4} = \left|\begin{matrix}a_{11} & a_{12} & a_{13} & a_{14}\\a_{12} & a_{22} & a_{23} & a_{24}\\a_{13} & a_{23} & a_{33} & a_{34}\\a_{14} & a_{24} & a_{34} & a_{44}\end{matrix}\right|$$
$$I{\left(\lambda \right)} = \left|\begin{matrix}a_{11} - \lambda & a_{12} & a_{13}\\a_{12} & a_{22} - \lambda & a_{23}\\a_{13} & a_{23} & a_{33} - \lambda\end{matrix}\right|$$
sustituimos coeficientes
$$I_{1} = 0$$
$$I_{3} = \left|\begin{matrix}0 & \frac{1}{2} & \frac{1}{2}\\\frac{1}{2} & 0 & \frac{1}{2}\\\frac{1}{2} & \frac{1}{2} & 0\end{matrix}\right|$$
$$I_{4} = \left|\begin{matrix}0 & \frac{1}{2} & \frac{1}{2} & 0\\\frac{1}{2} & 0 & \frac{1}{2} & 0\\\frac{1}{2} & \frac{1}{2} & 0 & 0\\0 & 0 & 0 & 0\end{matrix}\right|$$
$$I{\left(\lambda \right)} = \left|\begin{matrix}- \lambda & \frac{1}{2} & \frac{1}{2}\\\frac{1}{2} & - \lambda & \frac{1}{2}\\\frac{1}{2} & \frac{1}{2} & - \lambda\end{matrix}\right|$$
$$I_{1} = 0$$
$$I_{2} = - \frac{3}{4}$$
$$I_{3} = \frac{1}{4}$$
$$I_{4} = 0$$
$$I{\left(\lambda \right)} = - \lambda^{3} + \frac{3 \lambda}{4} + \frac{1}{4}$$
$$K_{2} = 0$$
$$K_{3} = 0$$
Como
entonces por razón de tipos de rectas:
hay que
Formulamos la ecuación característica para nuestra superficie:
$$- I_{1} \lambda^{2} + I_{2} \lambda - I_{3} + \lambda^{3} = 0$$
o
$$\lambda^{3} - \frac{3 \lambda}{4} - \frac{1}{4} = 0$$
$$\lambda_{1} = 1$$
$$\lambda_{2} = - \frac{1}{2}$$
$$\lambda_{3} = - \frac{1}{2}$$
entonces la forma canónica de la ecuación será
$$\left(\tilde a^{2} \lambda_{3} + \left(\tilde b^{2} \lambda_{2} + \tilde c^{2} \lambda_{1}\right)\right) + \frac{I_{4}}{I_{3}} = 0$$
$$- \frac{\tilde a^{2}}{2} - \frac{\tilde b^{2}}{2} + \tilde c^{2} = 0$$
$$- \frac{\tilde c^{2}}{1^{2}} + \left(\frac{\tilde a^{2}}{\left(\sqrt{2}\right)^{2}} + \frac{\tilde b^{2}}{\left(\sqrt{2}\right)^{2}}\right) = 0$$
es la ecuación para el tipo cono
- está reducida a la forma canónica
$$a b + a c + b c = 0$$
Esta ecuación tiene la forma:
$$a^{2} a_{33} + 2 a a_{13} c + 2 a a_{23} b + 2 a a_{34} + a_{11} c^{2} + 2 a_{12} b c + 2 a_{14} c + a_{22} b^{2} + 2 a_{24} b + a_{44} = 0$$
donde
$$a_{11} = 0$$
$$a_{12} = \frac{1}{2}$$
$$a_{13} = \frac{1}{2}$$
$$a_{14} = 0$$
$$a_{22} = 0$$
$$a_{23} = \frac{1}{2}$$
$$a_{24} = 0$$
$$a_{33} = 0$$
$$a_{34} = 0$$
$$a_{44} = 0$$
Las invariantes de esta ecuación al transformar las coordenadas son los determinantes:
$$I_{1} = a_{11} + a_{22} + a_{33}$$
|a11 a12| |a22 a23| |a11 a13|
I2 = | | + | | + | |
|a12 a22| |a23 a33| |a13 a33|$$I_{3} = \left|\begin{matrix}a_{11} & a_{12} & a_{13}\\a_{12} & a_{22} & a_{23}\\a_{13} & a_{23} & a_{33}\end{matrix}\right|$$
$$I_{4} = \left|\begin{matrix}a_{11} & a_{12} & a_{13} & a_{14}\\a_{12} & a_{22} & a_{23} & a_{24}\\a_{13} & a_{23} & a_{33} & a_{34}\\a_{14} & a_{24} & a_{34} & a_{44}\end{matrix}\right|$$
$$I{\left(\lambda \right)} = \left|\begin{matrix}a_{11} - \lambda & a_{12} & a_{13}\\a_{12} & a_{22} - \lambda & a_{23}\\a_{13} & a_{23} & a_{33} - \lambda\end{matrix}\right|$$
|a11 a14| |a22 a24| |a33 a34|
K2 = | | + | | + | |
|a14 a44| |a24 a44| |a34 a44| |a11 a12 a14| |a22 a23 a24| |a11 a13 a14|
| | | | | |
K3 = |a12 a22 a24| + |a23 a33 a34| + |a13 a33 a34|
| | | | | |
|a14 a24 a44| |a24 a34 a44| |a14 a34 a44|sustituimos coeficientes
$$I_{1} = 0$$
| 0 1/2| | 0 1/2| | 0 1/2|
I2 = | | + | | + | |
|1/2 0 | |1/2 0 | |1/2 0 |$$I_{3} = \left|\begin{matrix}0 & \frac{1}{2} & \frac{1}{2}\\\frac{1}{2} & 0 & \frac{1}{2}\\\frac{1}{2} & \frac{1}{2} & 0\end{matrix}\right|$$
$$I_{4} = \left|\begin{matrix}0 & \frac{1}{2} & \frac{1}{2} & 0\\\frac{1}{2} & 0 & \frac{1}{2} & 0\\\frac{1}{2} & \frac{1}{2} & 0 & 0\\0 & 0 & 0 & 0\end{matrix}\right|$$
$$I{\left(\lambda \right)} = \left|\begin{matrix}- \lambda & \frac{1}{2} & \frac{1}{2}\\\frac{1}{2} & - \lambda & \frac{1}{2}\\\frac{1}{2} & \frac{1}{2} & - \lambda\end{matrix}\right|$$
|0 0| |0 0| |0 0|
K2 = | | + | | + | |
|0 0| |0 0| |0 0| | 0 1/2 0| | 0 1/2 0| | 0 1/2 0|
| | | | | |
K3 = |1/2 0 0| + |1/2 0 0| + |1/2 0 0|
| | | | | |
| 0 0 0| | 0 0 0| | 0 0 0|$$I_{1} = 0$$
$$I_{2} = - \frac{3}{4}$$
$$I_{3} = \frac{1}{4}$$
$$I_{4} = 0$$
$$I{\left(\lambda \right)} = - \lambda^{3} + \frac{3 \lambda}{4} + \frac{1}{4}$$
$$K_{2} = 0$$
$$K_{3} = 0$$
Como
I3 != 0
entonces por razón de tipos de rectas:
hay que
Formulamos la ecuación característica para nuestra superficie:
$$- I_{1} \lambda^{2} + I_{2} \lambda - I_{3} + \lambda^{3} = 0$$
o
$$\lambda^{3} - \frac{3 \lambda}{4} - \frac{1}{4} = 0$$
$$\lambda_{1} = 1$$
$$\lambda_{2} = - \frac{1}{2}$$
$$\lambda_{3} = - \frac{1}{2}$$
entonces la forma canónica de la ecuación será
$$\left(\tilde a^{2} \lambda_{3} + \left(\tilde b^{2} \lambda_{2} + \tilde c^{2} \lambda_{1}\right)\right) + \frac{I_{4}}{I_{3}} = 0$$
$$- \frac{\tilde a^{2}}{2} - \frac{\tilde b^{2}}{2} + \tilde c^{2} = 0$$
$$- \frac{\tilde c^{2}}{1^{2}} + \left(\frac{\tilde a^{2}}{\left(\sqrt{2}\right)^{2}} + \frac{\tilde b^{2}}{\left(\sqrt{2}\right)^{2}}\right) = 0$$
es la ecuación para el tipo cono
- está reducida a la forma canónica