Virginia SOL Math / Grade 5 / Measurement & Geometry

5.MG.3

Classify & Measure Angles and Triangles

What you'll learn

  • Classify and measure angles as right, acute, obtuse, or straight
  • Classify triangles by their sides and angles
  • Find an unknown angle using known angle relationships

Words to know

Angle —
the figure formed by two rays sharing an endpoint
Acute angle —
an angle less than 90 degrees
Obtuse angle —
an angle greater than 90 but less than 180 degrees
Right angle —
an angle of exactly 90 degrees
Equilateral triangle —
a triangle with three equal sides

Why it matters Angles and triangles are the building blocks of designs and structures, so measuring and naming them helps us build and describe shapes.

Worked examples

Model ItI do

Classify an angle that measures 120 degrees.

A right angle is 90 degrees and a straight angle is 180 degrees. Since 120 is between 90 and 180, the angle is obtuse.

Try It TogetherWe do

Two angles of a triangle are 50 and 60 degrees. Find the third angle.

The three angles of a triangle add to 180 degrees. Add 50 + 60 = 110, then subtract: 180 - 110 = 70 degrees.

Now YouYou do

A triangle has angles of 90 and 45 degrees. Find the third angle.

Use the fact that all three angles sum to 180 degrees. Check your answer in the back of this chapter.

Practice

Mastered 0 of 11

Warm-up

Ease in with the basics.

  1. 1.Classify an angle of 30 degrees.

  2. 2.Classify an angle of 180 degrees.

  3. 3.A triangle has three equal angles. What type is it?

On Level

The core Quick Check from the book.

  1. 1.Classify an angle of 45 degrees.

  2. 2.Classify an angle of 90 degrees.

  3. 3.Two angles of a triangle are 70 and 60 degrees. Find the third.

  4. 4.What is the sum of the angles in any triangle?

  5. 5.A triangle has three equal sides. What is it called?

Challenge

Stretch your thinking.

  1. 1.Two angles of a triangle are 105 and 25 degrees. Find the third angle and classify the triangle by its angles.

  2. 2.A right triangle has one angle of 37 degrees. Find the other two angles.

  3. 3.An isosceles triangle has a top angle of 40 degrees. Find each of the two equal base angles.