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Parallel Line Theorem

When a line (transversal) crosses two parallel lines, 8 angles are formed. Since the two parallel lines are going in the same direction, the transversal crosses them in the same way. By the opposite angle theorem, we end up with two groups of 4 equal angles. Angles from different groups are supplementary. In the sketch, you can move the blue points (A, G, H). The equal angles are marked with the same marking and colour.