Question #293621

Show that the conicoid 2𝑥2 + 2𝑦2 + 𝑥𝑦 − 𝑦𝑧 + 𝑧𝑥 + 2𝑥 − 𝑦 + 5𝑧 + 1 = 0 is

central. Hence find its centre.


1
Expert's answer
2022-02-11T12:56:25-0500

1] A conicoid, given by the equation

ax2+by2+cz2+2fyz+2gxz+2hxy+2ux+2vy+2wz+d=0ax^2+by^2+cz^2+2fyz+2gxz+2hxy+2ux+2vy+2wz+d=0

has the point P(x0, y0, z0)P(x_0,\ y_0,\ z_0) as a center if and only if;

ax0+hy0+gz0+u=0hx0+by0+fz0+v=0gx0+fy0+cz0+w=0ax_0+hy_0+gz_0+u=0\\ hx_0+by_0+fz_0+v=0\\ gx_0+fy_0+cz_0+w=0


Now, by comparing

ax2+by2+cz2+2fyz+2gxz+2hxy+2ux+2vy+2wz+d=0ax^2+by^2+cz^2+2fyz+2gxz+2hxy+2ux+2vy+2wz+d=0

with the given equation

2𝑥2+2𝑦2+𝑥𝑦𝑦𝑧+𝑧𝑥+2𝑥𝑦+5𝑧+1=02𝑥^2 + 2𝑦^2 + 𝑥𝑦 − 𝑦𝑧 + 𝑧𝑥 + 2𝑥 − 𝑦 + 5𝑧 + 1 = 0

we have;

a=2, b=2, c=0, f=12, g=12, h=12, u=1, v=12, w=52, d=1\displaystyle a=2,\ b=2,\ c=0,\ f=-\frac{1}{2},\ g=\frac{1}{2},\ h=\frac{1}{2},\ u=1,\ v=-\frac{1}{2},\ w=\frac{5}{2},\ d=1

Thus,

ax0+hy0+gz0+u=0hx0+by0+fz0+v=0gx0+fy0+cz0+w=0ax_0+hy_0+gz_0+u=0\\ hx_0+by_0+fz_0+v=0\\ gx_0+fy_0+cz_0+w=0

\Rightarrow

4x0+y0+z0=2x0+4y0z0=1x0y0+0z0=5\displaystyle 4x_0+y_0+z_0=-2\\ x_0+4y_0-z_0=1\\ x_0-y_0+0z_0=-5


x0=135, y0=125, z0=6\displaystyle \Rightarrow x_0=-\frac{13}{5},\ y_0=\frac{12}{5},\ z_0=6on solving the system of equations.

Thus, the center of the conicoid is P(135, 125, 6)\displaystyle P\left(-\frac{13}{5},\ \frac{12}{5},\ 6\right).


2] A conicoid is called a central conicoid if it has a unique center.

To check if the given conicoid has a unique center, we need to show that the system of linear equations;

4x0+y0+z0=2x0+4y0z0=1x0y0+0z0=5\displaystyle 4x_0+y_0+z_0=-2\\ x_0+4y_0-z_0=1\\ x_0-y_0+0z_0=-5\\


(411141110)(x0y0z0)=(215)\Rightarrow \begin{pmatrix} 4 & 1 & 1\\ 1 & 4 & -1\\ 1 & -1 & 0 \end{pmatrix} \begin{pmatrix} x_0 \\ y_0 \\ z_0 \end{pmatrix}= \begin{pmatrix} -2\\ 1\\ -5 \end{pmatrix} , in matrix form has a unique solution (exactly one solution) by showing that the determinant of the coefficient matrix is not equal to zero. That is we need to show that

4111411100\begin{vmatrix} 4 & 1 & 1\\ 1 & 4 & -1\\ 1 & -1 & 0 \end{vmatrix}\neq 0

Now,

411141110=100\begin{vmatrix} 4 & 1 & 1\\ 1 & 4 & -1\\ 1 & -1 & 0 \end{vmatrix}=-10\neq0

Thus the coefficient matrix is invertible and x0, y0, z0x_0,\ y_0,\ z_0 are unique from the properties of matrix, with

(x0, y0, z0)=(135, 125, 6)\displaystyle (x_0,\ y_0,\ z_0)=\left(-\frac{13}{5},\ \frac{12}{5},\ 6\right)

showing that the conicoid center is unique.


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