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

DEVELOPMENT LESSON.

Let the figure a b c be a triangle.

Produce the side a c to d.

By the last theorem, the angle b c d is equal to what angles of the triangle?

What angle must we add to these angles to make up the three angles of the triangle?

If we add the same angle to the angle b c d, what adjacent angles do we get?

Then the three angles of the triangle, a, b, and c, are equal to what two angles?

But the adjacent angles a c b and b c d are equal to what?

Then, because the three angles of the triangle, a, b, and c, and two right angles, are separately equal to the two adjacent angles c and b c d.

What new thing have you found out?