501 Geometry Questions

Naming Triangles by Their Angles

You just learned how triangles can be classified by their sides and now you
will learn how to classify triangles by their angles. Triangles can be classi-
fied by the sizeof their angles or by the congruenceof their angles. First let’s
look at how triangles are classified by the sizeof their angles.

1. Acute triangles have three acute angles; none of the angles are 90° or
   greater than 90°. ΔEOF below is an acute triangle.

Acute Triangle EOF m∠EOF, m∠OFE

and m∠FEO < 90°

2. Obtuse triangleshave one angle that is obtuse, or greater than 90°, and
   two acute angles. (Remember, it is impossible for a triangle to have more
   than one obtuse angle because the sum of the interior angles of a trian-
   gle always equals 180°.) The following ΔMOL is an obtuse triangle.

● Obtuse Triangle LMO

● m∠LOM > 90°

● m∠OLM and m∠LMO < 90°

24 °

140 ° 16 °

M

O L

E

F O

86 °

54 ° 40 °

501 Geometry Questions