## Proof Without Words — Intersecting Planes Always Contain Perpendicular Lines

1. Ravi says:

When I say to my friends I maths can be beautiful and elegant they look at me funny – but this is exactly the kind of thing I mean.

• MrHonner says:

Thanks, Ravi. I was thinking about a simple way to understand this fact and I worked my way through line of reasoning. I’m pretty happy with how it came out.

Yes, I know the feeling–not everyone is able to really appreciate mathematics. But that’s why we keep trying, right?

2. Nat says:

The “proof” shows more; that is, there are infinitely many such perpendicular lines. But isn’t it being assumed that the dihedral angles are continuously changing and doesn’t that have to be proved? Anyway, it is a wonderful “plausibility argument.”

• MrHonner says:

This argument definitely relies on the continuity of that changing angle. As a naturally defined function, I might argue that it’s up to you to show it’s not continuous.

3. Nat says:

Taking a cue from your work, it is relatively easy to construct an analytic proof. Let z = 0 be one plane and z = m*y be the other. Without loss of generality, take m > 0. It seems arctan(m) is the dihedral angle between the planes, which intersect along the x-axis. Consider the line given by y = x in R^2, which passes through the origin and lies in z = 0. One way to parametrize it is: x = t = y and z = 0. A vector in the direction of the line is . A line perpendicular to it that lies in z = m*y and also passes through the origin is given by: x = -s, y = s, z = m*s. A vector in the direction of this line is . The vectors are perpendicular, since their dot product is zero.