Se da la ecuación de superficie de 2 grado:
$$x^{2} - 4 x y + 4 x z - 2 y^{2} + 8 y z - 2 z^{2} = 0$$
Esta ecuación tiene la forma:
$$a_{11} x^{2} + 2 a_{12} x y + 2 a_{13} x z + 2 a_{14} x + a_{22} y^{2} + 2 a_{23} y z + 2 a_{24} y + a_{33} z^{2} + 2 a_{34} z + a_{44} = 0$$
donde
$$a_{11} = 1$$
$$a_{12} = -2$$
$$a_{13} = 2$$
$$a_{14} = 0$$
$$a_{22} = -2$$
$$a_{23} = 4$$
$$a_{24} = 0$$
$$a_{33} = -2$$
$$a_{34} = 0$$
$$a_{44} = 0$$
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
$$I_{1} = -3$$
|1 -2| |-2 4 | |1 2 |
I2 = | | + | | + | |
|-2 -2| |4 -2| |2 -2|
$$I_{3} = \left|\begin{matrix}1 & -2 & 2\\-2 & -2 & 4\\2 & 4 & -2\end{matrix}\right|$$
$$I_{4} = \left|\begin{matrix}1 & -2 & 2 & 0\\-2 & -2 & 4 & 0\\2 & 4 & -2 & 0\\0 & 0 & 0 & 0\end{matrix}\right|$$
$$I{\left(\lambda \right)} = \left|\begin{matrix}1 - \lambda & -2 & 2\\-2 & - \lambda - 2 & 4\\2 & 4 & - \lambda - 2\end{matrix}\right|$$
|1 0| |-2 0| |-2 0|
K2 = | | + | | + | |
|0 0| |0 0| |0 0|
|1 -2 0| |-2 4 0| |1 2 0|
| | | | | |
K3 = |-2 -2 0| + |4 -2 0| + |2 -2 0|
| | | | | |
|0 0 0| |0 0 0| |0 0 0|
$$I_{1} = -3$$
$$I_{2} = -24$$
$$I_{3} = -28$$
$$I_{4} = 0$$
$$I{\left(\lambda \right)} = - \lambda^{3} - 3 \lambda^{2} + 24 \lambda - 28$$
$$K_{2} = 0$$
$$K_{3} = 0$$
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
$$\lambda^{3} + 3 \lambda^{2} - 24 \lambda + 28 = 0$$
$$\lambda_{1} = -7$$
$$\lambda_{2} = 2$$
$$\lambda_{3} = 2$$
entonces la forma canónica de la ecuación será
$$\left(\tilde z^{2} \lambda_{3} + \left(\tilde x^{2} \lambda_{1} + \tilde y^{2} \lambda_{2}\right)\right) + \frac{I_{4}}{I_{3}} = 0$$
$$- 7 \tilde x^{2} + 2 \tilde y^{2} + 2 \tilde z^{2} = 0$$
$$- \frac{\tilde x^{2}}{\left(\frac{\sqrt{7}}{7}\right)^{2}} + \left(\frac{\tilde y^{2}}{\left(\frac{\sqrt{2}}{2}\right)^{2}} + \frac{\tilde z^{2}}{\left(\frac{\sqrt{2}}{2}\right)^{2}}\right) = 0$$
es la ecuación para el tipo cono
- está reducida a la forma canónica