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

Diagram 37.

PROPOSITION III. THEOREM.

DEVELOPMENT LESSON.

In the above diagram, the lines a b, c d, are parallel, and are intersected by the line e f at the points m and n.

The angle Red measures the difference of direction between the line m b and what other line?

The angle Yellow measures the difference of direction between the line n d and what other line?

Then, as the lines m b and n d are parallel, must there not be the same difference of direction between them and the line e f?

Then can there be any difference between the angles which measure those equal directions?

Then what do you think of the opposite exterior and interior angles Red, Yellow?