Acute, Obtuse, and Right Triangles
This is a free geometry lesson for 4th grade about acute, obtuse, and right triangles (classification according to angles). It contains varied exercises, including several where students explore these concepts  and even the angle sum of a triangle  by drawing.
right angles 
obtuse angles 
acute angles 

Obtuse triangles have 
Acute triangles have three 
1. a. Draw a right angle.
Then b. Draw another,
different
c. A right
triangle has one right


A right triangle has one right angle. The other two angles are _____________________. 
2. a. Draw an obtuse angle.


b. Draw another,
different c. An
obtuse triangle has one 
An obtuse triangle has one obtuse angle. The other two angles are _____________________. 
3. a. Draw an acute triangle.
b. Measure its angles.

4. Observe all you have done thus far in this lesson, and fill in.
Right triangles have exactly 1 _________________ ________________, and the other two angles are _________________ . Obtuse triangles have exactly 1 _________________ ________________, and the other two angles are _________________ . Acute triangles have ___ ____________ angles. 
5. Label the triangles in the pictures as right, acute, or obtuse.
a. 
b. 
c. 
d. 
e. 
f. 
g. 
7. a. Draw a triangle with
Hint: First draw a 85° angle. b.
Measure the third angle. c. What kind of triangle
is it? d. What is the angle sum? 

b.
Measure the third angle. c. What kind of triangle is it? d. What is the angle sum? 

9. a. Draw a triangle with b. Measure the third
angle. c. What kind of triangle is it? d. What is the angle sum?

This lesson is taken from my book Math Mammoth Geometry 1.
Math Mammoth Geometry 1
A selfteaching worktext for 4th5th grade that covers angles, triangles, quadrilaterals, cirlce, symmetry, perimeter, area, and volume. Lots of drawing exercises!
Download ($6.90). Also available as a printed copy.