Common Section of Planes Perpendicular to other Plane is Perpendicular to that Plane
Theorem
In the words of Euclid:
- If two planes which cut one another be at right angles to any plane, their common section will also be at right angles to the same plane.
(The Elements: Book $\text{XI}$: Proposition $19$)
Proof
Let $AB$ and $BC$ be planes which are perpendicular to the plane of reference.
Let $BD$ be the common section of $AB$ and $BC$.
It is to be demonstrated that $BD$ is perpendicular to the plane of reference.
Suppose $BD$ is not perpendicular to the plane of reference.
From the point $D$, let:
- $DE$ be drawn in the plane $AB$ perpendicular to $AD$
- $DF$ be drawn in the plane $AC$ perpendicular to $CD$
We have that:
- $AB$ is perpendicular to the plane of reference
and:
- $DE$ is in the plane $AB$ perpendicular to $AD$.
Therefore from Book $\text{XI}$ Definition $4$: Plane at Right Angles to Plane:
- $DE$ is perpendicular to the plane of reference.
Similarly it can be proved that $DF$ is also perpendicular to the plane of reference.
Therefore from $D$ we have two straight lines have been set up perpendicular to the plane of reference on the same side.
This contradicts Proposition $13$ of Book $\text{XI} $: Straight Line Perpendicular to Plane from Point is Unique.
Hence the result.
$\blacksquare$
Historical Note
This proof is Proposition $19$ of Book $\text{XI}$ of Euclid's The Elements.
Sources
- 1926: Sir Thomas L. Heath: Euclid: The Thirteen Books of The Elements: Volume 3 (2nd ed.) ... (previous) ... (next): Book $\text{XI}$. Propositions