Angles with Parallel or Perpendicular Arms are Equal or Supplementary

Theorem

Let $A$ and $B$ be angles such that their corresponding arms are either parallel or perpendicular.

Then $A$ and $B$ are either equal or supplementary.

Proof

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Sources

• 1968: M.N. Aref and William Wernick: Problems & Solutions in Euclidean Geometry ... (previous) ... (next): Chapter $1$: Triangles and Polygons: Theorems and Corollaries $1.6$: Corollary