If L, M and N are three straight lines such that L || M and L || N, then prove that M|| N.
L || M M|| L.
Now, M||L and L||N M||N.
Assume M will intersect N.
It is given that L ||M and L ||N
According to our assumption, M will intersect N which is contradictory to parallel axiom.
So, our assumption is wrong.
Thus, M|| N