Cosine rule is another trigonometry rule that allows you to find missing sides and angles of triangles. This rule is used when we know an angle in between two angles or when we know 3 sides of the triangle. There are several different forms of this rule as you can see on the right. The top ones are for finding missing sides while the bottom ones are for finding missing angles.
Finding Missing Sides Using Cosine Rule
To use cosine rule we first have to annotate the sides and angles so that A, B & C are the angles and a, b and c the sides opposite to the corresponding angle. Then you have to use the correct rule. In this case we want to find the missing side a which is why we use the rule starting with a². Now simply substitute the values from the triangle into the formula and then solve the individual parts. Pay attention to the order of operation. Use your calculator to find a² and then square root your answer to find the missing side.
Finding Missing Angles Using Cosine Rule
In order to find a missing angle using cosine rule, we need to know all the sides of the triangle. Again, it is wise to annotate the sides and angles. In the example on the left we have to use the Cos B rule since the missing angle is B. To solve the equation simply substitute in the values and then simplify the equation until you are left with a simple numerical fraction. Since we want to find B and nod Cos B put this fraction into the inverse cosine function of your calculator. This will give you the size of the unknown angle. In this case, it is 35.5°. Have a look at a couple of examples underneath.
Example 1In this example, we have used cosine rule to find the missing angle A. We have substituted the values into the equation and simplified it before using the inverse cosine function on the calculator to find the missing angle.
Example 2In this example, we have used cosine rule to find the missing side c of the triangle. We have substituted the values into the equation and simplified it before square rooting 451 to find the length of side c: 21.2 cm.
Learn More Maths
You should now master all the topics of trigonometry and be able to move on to another challenging and interesting topic. You can visit our library to browse all lessons on our website. Please share this page if you like it or found it helpful!
ULTIMATE MATHSBecoming an Accomplished MathematicianUltimate Maths is a professional maths website that gives students the opportunity to learn, revise and apply different maths skills. We provide a wide range of lessons and resources...
Quality ContentA wide range of quality learning resources is at your disposal.
Effective TeachingExplanations, examples and questions combined for an effective learning experience.
Easy NavigationA simple user interface ensures that you find the topics you are looking for.
Excellent SupportOur fast and reliable support answer all your questions to your satisfaction.
Chapter 31.3:Learning OutcomesStudents will learn how to find missing angles of non right-angled triangles using cosine rule!Students will learn how to find missing sides of non right-angled triangles using cosine rule!