A conicoid given by equation ax2 +by2 +cz2 +2fyz+2gxz+2hxy+2ux+2vy+2wz+d=0 has a point (x0 ,y0 ,z0 ) as a center if
{ a x 0 + h y 0 + g z 0 + u = 0 h x 0 + b y 0 + f z 0 + v = 0 g x 0 + f y 0 + c z 0 + w = 0 \begin{cases}
ax_
0
+hy_0
+gz_
0
+u=0 &\ \\
hx_
0
+by_
0
+fz_
0
+v=0 \\
gx_
0
+fy_0
+cz_
0+w=0\\
\end{cases} ⎩ ⎨ ⎧ a x 0 + h y 0 + g z 0 + u = 0 h x 0 + b y 0 + f z 0 + v = 0 g x 0 + f y 0 + c z 0 + w = 0
1)
a = b = c = 1 , u = v = w = 1 / 2 , d = − 1 , f = g = h = 0 a=b=c=1, u=v=w=1/2, d=-1, f=g=h=0 a = b = c = 1 , u = v = w = 1/2 , d = − 1 , f = g = h = 0
{ x 0 + 1 / 2 = 0 y 0 + 1 / 2 = 0 z 0 + 1 / 2 = 0 \begin{cases}
x_
0
+1/2=0 \\
y_
0
+1/2=0 \\
z_
0
+1/2=0\\
\end{cases} ⎩ ⎨ ⎧ x 0 + 1/2 = 0 y 0 + 1/2 = 0 z 0 + 1/2 = 0
This is the central conicoid with a center (-1/2,-1/2,-1/2)
2)
a = 2 , h = 2 , g = 1 / 2 , u = − 1 / 2 , v = − 3 / 2 , w = 5 / 2 , d = 3 , b = c = f = 0 a=2, h=2, g=1/2, u=-1/2, v=-3/2, w=5/2, d=3, b=c=f=0 a = 2 , h = 2 , g = 1/2 , u = − 1/2 , v = − 3/2 , w = 5/2 , d = 3 , b = c = f = 0
{ 2 x 0 + 2 y 0 + z 0 / 2 − 1 / 2 = 0 2 x 0 − 3 / 2 = 0 x 0 / 2 + 5 / 2 = 0 \begin{cases}
2x_0
+2y_0
+z_
0
/2−1/2=0 \\
2x_
0
−3/2=0 \\
x_
0/2+
5/2=0\\
\end{cases} ⎩ ⎨ ⎧ 2 x 0 + 2 y 0 + z 0 /2 − 1/2 = 0 2 x 0 − 3/2 = 0 x 0 /2 + 5/2 = 0
This is not a central conicoid.
3)
a = 1 , b = c = − 1 , h = 1 / 2 , f = 2 , u = 1 / 2 , d = g = v = w = 0 a=1, b=c=-1, h=1/2, f=2, u=1/2, d=g=v=w=0 a = 1 , b = c = − 1 , h = 1/2 , f = 2 , u = 1/2 , d = g = v = w = 0
{ x 0 + y 0 / 2 + 1 / 2 = 0 x 0 / 2 − y 0 + 2 z 0 = 0 2 y 0 − z 0 = 0 \begin{cases}
x_
0
+y_
0
/2+1/2=0 \\
x_
0
/2−y_
0
+2z_
0
=0\\
2y_0
-z_0=0\\
\end{cases} ⎩ ⎨ ⎧ x 0 + y 0 /2 + 1/2 = 0 x 0 /2 − y 0 + 2 z 0 = 0 2 y 0 − z 0 = 0
{ x 0 = − 1 / 2 − z 0 / 4 − 1 / 4 − z 0 / 8 − z 0 / 2 + 2 z 0 = 0 y 0 = z 0 / 2 \begin{cases}
x_0
=−1/2−z_
0
/4 \\
−1/4−z_
0
/8−z_
0
/2+2z_
0
=0\\
y_0=z_0/2
\\
\end{cases} ⎩ ⎨ ⎧ x 0 = − 1/2 − z 0 /4 − 1/4 − z 0 /8 − z 0 /2 + 2 z 0 = 0 y 0 = z 0 /2
{ x 0 = − 9 / 11 z 0 = 2 / 11 y 0 = 1 / 11 \begin{cases}
x_0=-9/11
\\
z_0=2/11
\\
y_0=1/11
\\
\end{cases} ⎩ ⎨ ⎧ x 0 = − 9/11 z 0 = 2/11 y 0 = 1/11
This is the central conicoid with a center (-6/11,1/11,2/11)
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