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

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

Read three other acute-angled triangles.

OBTUSE-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.

RIGHT-ANGLED TRIANGLES.

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

Then it is called a right-angled triangle.