Grade 4 Geometry: Classifying Triangles

Worked examples

Real problems from the practice worksheets, solved one step at a time.

1. Example 1

   A triangle has sides of 9 cm, 9 cm, and 9 cm. Is it equilateral, isosceles, or scalene?

   1. By sides: equilateral = 3 equal sides, isosceles = exactly 2 equal sides, scalene = no equal sides.
   2. Compare the sides: 9 cm, 9 cm, 9 cm. How many of them are equal?
   3. So the triangle is equilateral.

   Answer: equilateral

2. Example 2

   A triangle has angles of 40°, 75°, and 65°. Is it acute, right, or obtuse?

   1. By angles: a right triangle has a 90° angle, an obtuse triangle has an angle greater than 90°, and an acute triangle has 3 angles less than 90°.
   2. Find the largest angle of 40°, 75°, and 65°. Is it less than, equal to, or greater than 90°?
   3. So the triangle is acute.

   Answer: acute

3. Example 3

   How many acute angles does an acute triangle have?

   1. By angles: a right triangle has a 90° angle, an obtuse triangle has an angle greater than 90°, and an acute triangle has 3 angles less than 90°.
   2. In an acute triangle, every angle is less than 90°. How many angles does a triangle have?
   3. So the answer is 3.

   Answer: 3