I'm a bit confused in the methods that could help to solve this. The problem is the following:
In the ellipsoid: $$4x^2+y^2+4z^2-16x-6y-8z+25=0$$ find the furthest point and the nearest point of the plane: $$2x+2y+z=0$$
I think that using the method of Lagrange multipliers wil solve this, but I don't even know how to start. Can I give a suggestion for this? Please, and thanks.
Best Answer
The vector normal to the plane $2x+2y+z=0$ is (2,2,1). The normal vector to the ellipsoid at point $(x,y,z)$ is $(z_x',z_y',-1)$, or explicitly,
$$\left(-\frac{x-2}{z-1}, -\frac{y-3}{4z-4},-1\right)$$
For the nearest and furthest points from the plane, the normal vectors at these two points are parallel to the normal vector of the plane, i.e.
$$-\frac{x-2}{z-1}:-\frac{y-3}{4z-4}:-1 = 2:2:1$$
which leads to,
$$x=2z,\>\>\>\>\>y=8z-5\tag{1}$$
Substitute (1) into the equation of the ellipsoid $4x^2+y^2+4z^2-16x-6y-8z+25=0$ to get
$$21z^2-42z+20=0$$
which yields the coordinates $z=1\pm\frac 1{\sqrt{21}}$. Plug $z$ back into (1) to obtained the nearest and furthest point at
$$\left(2\pm\frac 2{\sqrt{21}},3\pm\frac 8{\sqrt{21}},1\pm\frac 1{\sqrt{21}}\right)$$