Marks' first lessons in geometry / In two parts. Objectively presented, and designed for the use of primary classes in grammar schools, academies, etc.

by Bernhard Marks

Place a paper square corner or right angle on the line a b at the left of c d with its vertex at c.

It will cover all the angle Green and part of the angle Red up to the line c d.

Now place another square corner on the line a b to the right of the line c d, and with its vertex at the point c.

It will cover the remaining part of the angle Red, and two edges of the square corners will meet along the line c d.

Are the two right angles equal to all the angular space on the line a b?

Then if the two adjacent angles Green, Red, are equal to all the angular space on the line a b, and the two right angles are also equal to the same space, what do you infer concerning the adjacent angles and the two right angles?

What axiom do you apply when you say that the adjacent angles are equal to the two right angles?

To what same thing did you find two things separately equal?

What did you first see equal to it?

What did you next see equal to it?