Se da la ecuación de superficie de 2 grado:
x 2 + 4 x y − 8 x z − 14 x − 2 y 2 − 4 y z − 4 y + z 2 + 14 z + 16 = 0 x^{2} + 4 x y - 8 x z - 14 x - 2 y^{2} - 4 y z - 4 y + z^{2} + 14 z + 16 = 0 x 2 + 4 x y − 8 x z − 14 x − 2 y 2 − 4 yz − 4 y + z 2 + 14 z + 16 = 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 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 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 yz + 2 a 24 y + a 33 z 2 + 2 a 34 z + a 44 = 0 donde
a 11 = 1 a_{11} = 1 a 11 = 1 a 12 = 2 a_{12} = 2 a 12 = 2 a 13 = − 4 a_{13} = -4 a 13 = − 4 a 14 = − 7 a_{14} = -7 a 14 = − 7 a 22 = − 2 a_{22} = -2 a 22 = − 2 a 23 = − 2 a_{23} = -2 a 23 = − 2 a 24 = − 2 a_{24} = -2 a 24 = − 2 a 33 = 1 a_{33} = 1 a 33 = 1 a 34 = 7 a_{34} = 7 a 34 = 7 a 44 = 16 a_{44} = 16 a 44 = 16 Las invariantes de esta ecuación al transformar las coordenadas son los determinantes:
I 1 = a 11 + a 22 + a 33 I_{1} = a_{11} + a_{22} + a_{33} I 1 = a 11 + a 22 + a 33 |a11 a12| |a22 a23| |a11 a13|
I2 = | | + | | + | |
|a12 a22| |a23 a33| |a13 a33| I 3 = ∣ a 11 a 12 a 13 a 12 a 22 a 23 a 13 a 23 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 3 = a 11 a 12 a 13 a 12 a 22 a 23 a 13 a 23 a 33 I 4 = ∣ 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 ∣ 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 4 = 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 I ( λ ) = ∣ a 11 − λ a 12 a 13 a 12 a 22 − λ a 23 a 13 a 23 a 33 − λ ∣ 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| I ( λ ) = a 11 − λ a 12 a 13 a 12 a 22 − λ a 23 a 13 a 23 a 33 − λ |a11 a14| |a22 a24| |a33 a34|
K2 = | | + | | + | |
|a14 a44| |a24 a44| |a34 a44| |a11 a12 a14| |a22 a23 a24| |a11 a13 a14|
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K3 = |a12 a22 a24| + |a23 a33 a34| + |a13 a33 a34|
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|a14 a24 a44| |a24 a34 a44| |a14 a34 a44| sustituimos coeficientes
I 1 = 0 I_{1} = 0 I 1 = 0 |1 2 | |-2 -2| |1 -4|
I2 = | | + | | + | |
|2 -2| |-2 1 | |-4 1 | I 3 = ∣ 1 2 − 4 2 − 2 − 2 − 4 − 2 1 ∣ I_{3} = \left|\begin{matrix}1 & 2 & -4\\2 & -2 & -2\\-4 & -2 & 1\end{matrix}\right| I 3 = 1 2 − 4 2 − 2 − 2 − 4 − 2 1 I 4 = ∣ 1 2 − 4 − 7 2 − 2 − 2 − 2 − 4 − 2 1 7 − 7 − 2 7 16 ∣ I_{4} = \left|\begin{matrix}1 & 2 & -4 & -7\\2 & -2 & -2 & -2\\-4 & -2 & 1 & 7\\-7 & -2 & 7 & 16\end{matrix}\right| I 4 = 1 2 − 4 − 7 2 − 2 − 2 − 2 − 4 − 2 1 7 − 7 − 2 7 16 I ( λ ) = ∣ 1 − λ 2 − 4 2 − λ − 2 − 2 − 4 − 2 1 − λ ∣ I{\left(\lambda \right)} = \left|\begin{matrix}1 - \lambda & 2 & -4\\2 & - \lambda - 2 & -2\\-4 & -2 & 1 - \lambda\end{matrix}\right| I ( λ ) = 1 − λ 2 − 4 2 − λ − 2 − 2 − 4 − 2 1 − λ |1 -7| |-2 -2| |1 7 |
K2 = | | + | | + | |
|-7 16| |-2 16| |7 16| |1 2 -7| |-2 -2 -2| |1 -4 -7|
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K3 = |2 -2 -2| + |-2 1 7 | + |-4 1 7 |
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|-7 -2 16| |-2 7 16| |-7 7 16| I 1 = 0 I_{1} = 0 I 1 = 0 I 2 = − 27 I_{2} = -27 I 2 = − 27 I 3 = 54 I_{3} = 54 I 3 = 54 I 4 = 0 I_{4} = 0 I 4 = 0 I ( λ ) = − λ 3 + 27 λ + 54 I{\left(\lambda \right)} = - \lambda^{3} + 27 \lambda + 54 I ( λ ) = − λ 3 + 27 λ + 54 K 2 = − 102 K_{2} = -102 K 2 = − 102 K 3 = 162 K_{3} = 162 K 3 = 162 Como
I3 != 0 entonces por razón de tipos de rectas:
hay que
Formulamos la ecuación característica para nuestra superficie:
− I 1 λ 2 + I 2 λ − I 3 + λ 3 = 0 - I_{1} \lambda^{2} + I_{2} \lambda - I_{3} + \lambda^{3} = 0 − I 1 λ 2 + I 2 λ − I 3 + λ 3 = 0 o
λ 3 − 27 λ − 54 = 0 \lambda^{3} - 27 \lambda - 54 = 0 λ 3 − 27 λ − 54 = 0 λ 1 = 6 \lambda_{1} = 6 λ 1 = 6 λ 2 = − 3 \lambda_{2} = -3 λ 2 = − 3 λ 3 = − 3 \lambda_{3} = -3 λ 3 = − 3 entonces la forma canónica de la ecuación será
( z ~ 2 λ 3 + ( x ~ 2 λ 1 + y ~ 2 λ 2 ) ) + I 4 I 3 = 0 \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 ( z ~ 2 λ 3 + ( x ~ 2 λ 1 + y ~ 2 λ 2 ) ) + I 3 I 4 = 0 6 x ~ 2 − 3 y ~ 2 − 3 z ~ 2 = 0 6 \tilde x^{2} - 3 \tilde y^{2} - 3 \tilde z^{2} = 0 6 x ~ 2 − 3 y ~ 2 − 3 z ~ 2 = 0 − x ~ 2 ( 6 6 ) 2 + ( y ~ 2 ( 3 3 ) 2 + z ~ 2 ( 3 3 ) 2 ) = 0 - \frac{\tilde x^{2}}{\left(\frac{\sqrt{6}}{6}\right)^{2}} + \left(\frac{\tilde y^{2}}{\left(\frac{\sqrt{3}}{3}\right)^{2}} + \frac{\tilde z^{2}}{\left(\frac{\sqrt{3}}{3}\right)^{2}}\right) = 0 − ( 6 6 ) 2 x ~ 2 + ( 3 3 ) 2 y ~ 2 + ( 3 3 ) 2 z ~ 2 = 0 es la ecuación para el tipo cono
- está reducida a la forma canónica