Given a triangle $\triangle $ ABC. Suppose there are two points inside the triangle, and they are $p$ and $q$. Let $d(p,q)=x$, here d represents distance. How to prove this:
- if $\min\{d(A,p), d(C,q)\}=d(A,p)$ then $d(A,q) \geq x$.
or
- if $\min\{d(A,p), d(C,q)\}=d(C,q)$ then $d(C,p) \geq x$.
In the figure, I showed the first if statement.

PS: I am doing it by drawing a circle centred at q and of radius x.
Any triangle that fails must have all its vertices inside the blue area, but any triangle like this can't contain points $p$ and $q$.