Marking Angles Using a Speed Square

The process for marking an angle using a speed square is shown below.

Swing the square until you read the desired angle (33-1/2 degree) on the board’s edge (photo top left), and make your mark. The line shown being marked (with pencil) is a 33-1/2 degree angle.Untitled

Math Standard: G.CO.9-10
Explain why this is true through a paragraph proof.  Use drawings to supplement your explanation.