Question #200478

Examine which of the following conicoids are central and which are non-central. Also determine which of the central conicoids have centre at the origin.

i) x2 +y2 +z2 + x + y + z = 1

ii) 2x2 +4xy+ xz−x−3y + 5z+ 3 = 0

iii) x2 −y2 −z2 +xy + 4yz + x = 0


1
Expert's answer
2021-06-01T16:29:38-0400

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

{ax0+hy0+gz0+u=0 hx0+by0+fz0+v=0gx0+fy0+cz0+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}


1)

a=b=c=1,u=v=w=1/2,d=1,f=g=h=0a=b=c=1, u=v=w=1/2, d=-1, f=g=h=0

{x0+1/2=0y0+1/2=0z0+1/2=0\begin{cases} x_ 0 ​ +1/2=0 \\ y_ 0 ​ +1/2=0 \\ z_ 0 ​ +1/2=0\\ \end{cases}

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=0a=2, h=2, g=1/2, u=-1/2, v=-3/2, w=5/2, d=3, b=c=f=0

{2x0+2y0+z0/21/2=02x03/2=0x0/2+​​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}

This is not a central conicoid.


3)

a=1,b=c=1,h=1/2,f=2,u=1/2,d=g=v=w=0a=1, b=c=-1, h=1/2, f=2, u=1/2, d=g=v=w=0

{x0+y0/2+1/2=0x0/2y0+2z0=02y0​​z0=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}

{x0=1/2z0/41/4z0/8z0/2+2z0=0y0=z0/2​​\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}

{x0=9/11z0=2/11​​y0=1/11​​\begin{cases} x_0=-9/11 \\ z_0=2/11 ​ ​ \\ y_0=1/11 ​ ​ \\ \end{cases}

This is the central conicoid with a center  (-6/11,1/11,2/11)


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