Originally Posted by

**Dr.Peterson**
What you describe is essentially what j-astron suggested doing. Nothing non-mathematical about it, really, unless you were told to prove it. An intuitive understanding like this is "mathematical", just in a different sense -- and vitally important.

If you want a proof, have you learned something that says that two intersecting lines determine a plane? If not, what theorems HAVE you learned about planes?

The same goes for the OP, of course; I'm treating you as a stand-in.

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