Se da la ecuación de superficie de 2 grado:
$$36 x^{2} + 9 y^{2} + 4 z^{2} - 36 = 0$$
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
$$a_{11} = 36$$
$$a_{12} = 0$$
$$a_{13} = 0$$
$$a_{14} = 0$$
$$a_{22} = 9$$
$$a_{23} = 0$$
$$a_{24} = 0$$
$$a_{33} = 4$$
$$a_{34} = 0$$
$$a_{44} = -36$$
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
$$I_{1} = 49$$
|36 0| |9 0| |36 0|
I2 = | | + | | + | |
|0 9| |0 4| |0 4|
$$I_{3} = \left|\begin{matrix}36 & 0 & 0\\0 & 9 & 0\\0 & 0 & 4\end{matrix}\right|$$
$$I_{4} = \left|\begin{matrix}36 & 0 & 0 & 0\\0 & 9 & 0 & 0\\0 & 0 & 4 & 0\\0 & 0 & 0 & -36\end{matrix}\right|$$
$$I{\left(\lambda \right)} = \left|\begin{matrix}36 - \lambda & 0 & 0\\0 & 9 - \lambda & 0\\0 & 0 & 4 - \lambda\end{matrix}\right|$$
|36 0 | |9 0 | |4 0 |
K2 = | | + | | + | |
|0 -36| |0 -36| |0 -36|
|36 0 0 | |9 0 0 | |36 0 0 |
| | | | | |
K3 = |0 9 0 | + |0 4 0 | + |0 4 0 |
| | | | | |
|0 0 -36| |0 0 -36| |0 0 -36|
$$I_{1} = 49$$
$$I_{2} = 504$$
$$I_{3} = 1296$$
$$I_{4} = -46656$$
$$I{\left(\lambda \right)} = - \lambda^{3} + 49 \lambda^{2} - 504 \lambda + 1296$$
$$K_{2} = -1764$$
$$K_{3} = -18144$$
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
$$\lambda^{3} - 49 \lambda^{2} + 504 \lambda - 1296 = 0$$
$$\lambda_{1} = 36$$
$$\lambda_{2} = 9$$
$$\lambda_{3} = 4$$
entonces la forma canónica de la ecuación será
$$\left(\tilde z^{2} \lambda_{3} + \left(\tilde x^{2} \lambda_{1} + \tilde y^{2} \lambda_{2}\right)\right) + \frac{I_{4}}{I_{3}} = 0$$
$$36 \tilde x^{2} + 9 \tilde y^{2} + 4 \tilde z^{2} - 36 = 0$$
2 2 2
\tilde x \tilde y \tilde z
--------- + --------- + --------- = 1
2 2 2
/ 1 \ / 1 \ / 1 \
|-----| |-----| |-----|
\6*1/6/ \3*1/6/ \2*1/6/
es la ecuación para el tipo elipsoide
- está reducida a la forma canónica