I have this question:
$L_1$ and $L_2$ are lines, and $P$ is a plane. $L_1$ does not lie in $P$, but meets the plane in a point $p$. $L_2$ lies in $P$, but does not contain $p$. Does $L_1$ meet $L_2$? Which of the axioms of incidence are used in solving this question?
I think $L_1$ does not meet $L_2$ because $L_1$ and $L_2$ are not coplanar ?!
The axioms are:
- (line axiom) Through any two distinct point there is exactly one line,
- (plane axiom) Through any $3$-non collinear points there is exactly one plane
- (dimension axiom) Any line contains at least two distinct points, any plane contains at least two distinct lines, there are at least two distinct planes.
- (line-plane intersection) If two distinct points of a line lie in some plane $P$ then the whole line lies on P
- (the parallel axiom) Let $L$ be a line and $p$ be a point then there is exactly one line that passes through $p$ and is parallel to $L$.
- (plane plane intersection) If two distinct planes meet then their intersection is a line.
The intersection of a line $L_1$ with a plane $P$ in $\mathbb{R}^3$ is either empty $L_1 \cap P = \emptyset$, contains one point $L_1 \cap P = \{ p \}$ or infinite many points $L_1 \cap P = L_1$.
(This reasoning is directly from Linear Algebra, it translates into the solution of a system of linear equations, one equation expressing the plane, two equations expressing the line)
If $L_1$ lies not in $P$ and has at least one point in common you must have the case with one intersection point $p$ and you know there can not be another point from $L_1$ on $L_2$ as that one has to lie in $P$ as well and we have only one in $P$.
Update: Now let us try the axioms:
We apply $(A \Rightarrow B) \iff (\neg B \Rightarrow A)$: $$ (\exists p, q \in L: p \ne q \wedge p, q \in P ) \Rightarrow L \subset P \iff \\ \neg (L \subset P) \Rightarrow \neg (\exists p, q \in L: p \ne q \wedge p, q \in P ) \iff \\ \neg (L \subset P) \Rightarrow (\forall: p, q \in L: p = q \vee p \notin P \vee q \notin P) $$ The last means: If $L$ does not lie in $P$ then for all two distinct points of $L$ at least one of them is not in $P$.
As $L_1$ does not lie in $P$ and $p \in P$ we know from the above $q \notin P$.
This too allows at most one point of $L_1$ to be in $P$: If we had two distinct ones both from $P$, at least one of them had not be in $P$, contradiction.
Again: We know there can not be another point $q \in L_1$ on $L_2$ as that one has with $L_2$ to lie in $P$ as well and we have only one in $P$.