# how to do law of cosines The Law of Cosines (also called the Cosine Rule) says: c 2 = a 2 + b 2 鈭?2ab cos (C) It helps us solve some triangles. Let’s see how to use it. Example: How long is side c … ? Do some calculations: c2 = 64 + 121 鈭?176 0.798鈥?More calculations: c2 = 44.44… Answer: c = 6.67 How can you remember the formula?

## People also ask

• ### What is the law of cosine used for?

• Cosine Rule (Law of Cosines) The cosine rule, also known as the law of cosines, relates all 3 sides of a triangle with an angle of a triangle. It is most useful for solving for missing information in a triangle. For example, if all three sides of the triangle are known, the cosine rule allows one to find any of the angle measures.

• ### How do you use cosine law to find the third side?

• The cosine law is used to determine the third side of a triangle when we know the lengths of the other two sides and the angle between them. Can we use cosine law for all the triangles?

• ### How do you use the law of cosines to find length?

• In the last lesson we used the Law of Cosines to find the length of the third side of a triangle when you know the length of two sides and the angle in between. This law can also be used to find the measure of an angle when the three sides of a triangle are given. 1.

• ### What is the cosine rule for a triangle?

• It is also called the cosine rule. If ABC is a triangle, then as per the statement of cosine law, we have: a2 = b2 + c2 鈥?2bc cos 伪, where a,b, and c are the sides of triangle and 伪 is the angle between sides b and c. Similarly, if 尾 and 纬 are the angles between sides ca and ab, respectively, then according to the law of cosine, we have: