Why are vertically opposite angles equal?
Let us take 2 lines and create vertical angles at the intersection of those lines. I am going to call the pairs of vertical angles A, A’ and B and B’ where A and A’ are vertical and B and B’ are vertical. Now in your drawing notice that the sum of the degrees of A and B is 180 degrees, they are suplementary. But the sum of A and B’ is also 180 degrees. So this means that the measure of B is equal to the measure of B’. We can do the same with A and A’.
