Arithmetic Grade Five

Lesson One hundred and fifty-one: Interior Angles of Triangles

Angles are measured in degrees (they can also be measured in radians, but we won't deal with radians). A triangle has three vertices and the interior of each vertice forms an angle. The sum of the three interior angles of a triangle is always 180 degrees.

These are the sizes of the angles of the triangle to the right.

  1. 43 degrees
  2. 115 degrees
  3. 22 degrees
Since we know that the sum of the interior angles of a triangle is always 180 degrees, we can always figure out the size of the third angle providing that we know the size of the other two. Consider this example:
  1. 42 degrees
  2. 85 degrees

The missing angle will equal: 

   180 - 42 - 85 =
Solving this problem will yield 53 degrees as the size of the missing angle.
Here's another missing angle for you to figure out:
  1. 39 degrees
  2. 116 degrees
The solution: 

   180 - 39 - 116 = 25 degrees

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