This activity is also part of one or more other Books. Modifications will be visible in all these Books. Do you want to modify the original activity or create your own copy for this Book instead?
This activity was created by '{$1}'. Do you want to modify the original activity or create your own copy instead?
This activity was created by '{$1}' and you lack the permission to edit it. Do you want to create your own copy instead and add it to the book?
Coxeter- Theorem 2.23
2.23 - [i]If two lines have a common point, they are coplanar.[/i]
PROOF. If two lines have a common point [i]C[/i], we may name them [i]AC[/i], [i]BC[/i], and conclude that they lie in the plane [i]ABC[/i].