Question

Find the counterexample for

If two lines intersect, then any third coplanar line intersects both of them

Answer 1

Coplanar lines are lines that lie on the same plane.

Counterexample:

From the attached diagram you can see two lines AB and EF that have intersection in point G.

The third line CD is coplanar with lines AB and EF, but this line intersects only line EF (point H) and doesn't intersect line AB, because AB||CD.