Unit A: Geometry

Chapter 3: Trigonometry

Finding the Cosine of an Angle Using a Calculator


Before the cosine law is studied, it is important to review how to find the cosine of an angle using a calculator. The two most common modes for measuring angles are degrees (DEG) and radian (RAD). For Math 30-3, it is important that the calculator is in degree mode. Many calculators have a mode button to change the setting to degree mode. In non-graphing calculators, DEG will appear on the screen. If you need help setting your calculator to degree mode, contact your teacher.

It is important to make sure that the cosine of an angle can be calculated properly using a calculator. The two most common ways to find the cosine of an angle are

Method 1:
Step 1: Press .
Step 2: Type in the angle.
Step 3: Press the equals sign.
Method 2:
Step 1: Type in the angle.
Step 2: Press .
Step 3: Press the equals sign.

Test your calculator using cos 57°. Determine which method correctly calculates cos 57° on your calculator.

Method 1:
Step 1: Press .
Step 2: Type in 57.
Step 3: Press the equals sign.
Method 2:
Step 1: Type in 57.
Step 2: Press .
Step 3: Press the equals sign.

The correct method will result in the number 0.5446 appearing on your calculator.

Note: Cosine values are generally written to four decimal places.

The cosine of an angle is
  • between 0 and 1 if the angle is acute, that is, between 0° and 90°.
  • between â€“1 and 0 if the angle is obtuse, that is, between 90° and 180°.