Lesson Twenty-First — Triangles — Marks’ First Lessons in Geometry by Bernhard Marks

Lesson Twenty-First Triangles

In the triangle 1, what kind of an angle is b a c? a c b? c b a? (Diagram 17.)

Then it is called an acute-angled triangle.

An acute-angled triangle is one whose angles are all acute.

Read three other acute-angled triangles.

In the triangle 4, what kind of an angle is l k m?

Then it is called an obtuse-angled triangle.

An obtuse-angled triangle is one that has one obtuse angle.

Name two others.

In the triangle 3, what kind of an angle is g i j?

Then it is called a right-angled triangle.

A right-angled triangle is one that has one right angle.

Name three other right-angled triangles.

Upon which side does the triangle 3 seem to stand?

Then i j is called the base of the triangle.

What letter marks the vertex of the angle opposite the base?

Then the point g is said to be the vertex of the triangle.

If, in the triangle 7, we consider t v the base, what point is the vertex?

If v be considered the vertex, which side will be the base?

In the triangle 3, what side is opposite the right angle?

Then g j is called the hypothenuse of the triangle.

The hypothenuse of a triangle is the side opposite the right angle.

Read the hypothenuse of each of the triangles 5, 6, and 11.

Either side about the right angle may be considered the base.

Then the other side will be the perpendicular.

In the triangle 3, if i j is the base, which side is the perpendicular?

If g i be considered the base, which side is the perpendicular?

In triangle 5, if n o is the base, which side is the perpendicular?