First, conceptualize the solution set as a set points
.
A direct check shows that the origin is always in the solution set for any value of
, so will hunt for non-origin solutions only.
The origin is obviously the only point in the solution for
. So assume going forward that
.
Next, the equation is homogeneous... if
is a solution, then so is
for any
.
By homogeneity, given a non-origin point P that's a solution, every point on the line through the origin and P is a solution.
Thus the total solution set is entirely determined by the solutions which are on the unit sphere (and if none, then the solution set is just the origin).
The solution set the is union of the all points on the set of lines going through the origin and any point of the solution on the unit sphere.
Holding off on the unit sphere for a moment, use that
to eliminate the cross terms on the right-hand side.
Do that by adding
to both sides, getting
. Dividing both sides by
(recall assuming
) gives:
, and so
(*)
Now by (*), the left hand side must be non-negative, as must right-hand side's factor
. Thus the only solution will be the origin when
.
The solution set of
is
.
Thus the origin is the only solution for
.
Now, assume
is such that
(meaning
), and consider just the points on the unit sphere (
).
Take the square root of (*) and it reads
.
Letting
, that says that any solution point
on the unit sphere must also be on one of the two planes
and
.
Now if a solution
on the unit sphere (so
), then by homogenity, its antipodal point
is also a solution on the unit sphere.
So if that point is also on the plane
(meaning
), then
is a solution, and that satisfies
.
Likewise, if a solution
is on the unit sphere and in the plane
, then its antipodal point
is a solution on the unit sphere and in the plane
.
Therefore, to find all non-origin solutions, we only need to find the intersection of unit sphere and the plane
, since at the end we'll build all solutions by including all the points on all the lines through the origin and a point on that intersection. The solutions on the unit sphere intersect the plane
will be captured in that process.
Note that the case
gives
is the case where those planes,
and
, are the same plane... they've come together and met. Everything applies in that case as well.
In
, a plane intersect the unit sphere is either the empty set (no intersection), a single point (the plane is tangent to the sphere), or a circle (the plane slices through the sphere).
Thus if you draw the all lines through the origin the go through a plane intersect the unit sphere, you'll either get the empty set, or a single line, or a double cone.
Thus the final solution to the problem will either be (depending on
): just the origin (since the origin is always a solution!), or a single line through the origin, or a double cone through the origin.
Now work out the precise solutions for a given
:
##################################################
Again, consider
, and
, and look only for the points on the unit sphere intersect the plane
.
One of that plane's normal vectors is
. Let
. Notice that this doesn't vary with
.
Let the plane's point closest point to the origin be defined as
.
Then
for some
. (That's geometrically obvious and has a straightforward proof.)
Whether, and/or how, the plane intersects the unit sphere is determined entirely by
.
If
, then there's no intersection. If
, then there's a single point of intersection (the plane's tangent to the sphere). If
, then the intersection is a circle.
To find
, use that
is a point on the plane
.
Thus
, so
, so
,
, so
. (Note that
, as the geometric picture suggested.)
With the formula
(remember it's always assumed that
), can now classify solutions according to
.
############
There's no intersection when
, hence whenever
. Solving that for k gives
, and in that case the grand solution to the problem is just the origin.
That can be rolled into the "origin only" solution when
, so that: for
, the only solution to the main problem is just the origin (
).
Thus the grand solution is the origin only in the case where
############
There's only one point of intersection when
, so when
. That occurs only when
.
The single point of intersection between the unti sphere and the plane in that case is the point
.
Therefore the solution set when
is the line through the origin and that point
, so is the line
.
Letting
, that can be written parametrically without using the square root.
Thus the grand solution, a line, in the case where
, is given by
, or even more concisely written as the line x = y = z
############
The intersection is a circle when
, so when
and
, so when either
or
.
In the case
, the circle in 3-space formed by the plane intersect the unit sphere, will have center
, and a normal vector
(since it's on the plane with that normal vector).
Let
be the radius of that circle. Then by looking at the triangle from the origin, to
, to a point on the circle, it's a right triangle with hypotenuses 1 and legs
.
Therefore
.
To find unit vectors mutually orthogonal to
, find one unit vector orthogonal to it, and then take the cross product to find another (they'll form an orthonormal basis for
).
It's easy to find one vector orthogonal to it,
, so let
.
Then let
.
The points on the circle can be parameterized using sines and cosines, except instead of the classic x and y axes of
, it's using the
and
"axes" in
.
To get to a point on that circle, go to
and then move radius-times-cosine in the
(like classic x-axis) direction, then radius-times-sine in the
(like classic y-axis) direction.
The points on that circle will be given by:
for all
.
In coordinates, that's
, so
.
.
.
Thus the grand solution, the double cone, in the case where
or
, is given by