Sorry for a silly question,
I am trying to prove the fact of intersection of two segments on the plane.
For example, $(d_1,d_2)$ is the first segment, where $d_1$ and $d_2$ are endpoint of the segment, and $(d_3,d_4)$ is the second segment.
One of the ways to show the existence of intersection is to show that $d_1$ and $d_2$ are placed on the different sides of the line $l(d_3,d_4)$ and $d_3$, $d_4$ on the different sides of $l(d_1,d_2)$.
It's very intuitive statement and obviously this should be true, but how to prove this.
How to prove that if endpoints of the one segment are located on the different sides of the another segment and vice versa show there is a intersection of segments.