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

PROPOSITION X. THEOREM.

DEVELOPMENT LESSON.

Let the figure a b c be a triangle.

Produce the side a c to d.

We have now another angle, b c d, and we wish to find out if it is equal to any of the angles of the triangle.

From the point c draw the line c e parallel to a b.

Because the straight line a d intersects the two parallels a b, c e, the angle a is equal to what other angle?

Because the straight line b c intersects the two parallels a b, c e, the angle b is equal to what other angle?

Then the angles a and b are equal to what two angles?

How does the angle b c d compare with the angles b c e, e c d?