# Law of Cosines or Cosine Law

Related Topics & Worksheets:
Cosine Rule
Sine /Rule

Objective: I know how to use the Law of Cosines or Cosine Law to solve triangles.

The Law of Cosines, for any triangle *ABC* is

a 2 = b 2 + c 2 – 2bc cos A

Refer to the lesson on the [law or cosines or cosine law ](/law-of-cosines.html) for more information and examples.

Fill in all the gaps, then press "Check" to check your answers. Use the "Hint" button to get a free letter if an answer is giving you trouble. You can also click on the "[?]" button to get a clue. Note that you will lose points if you ask for hints or clues!
1. Find the length of the third side, to 3 decimal places, and the other two angles, to 1 decimal place, in the following triangles

(a) a = 1, b = 2, C = 30°
c = , A = °, B = °

(b) a = 3, c = 4, B = 50°
b = , A = °, C = °

(c) b = 5, c = 10, A = 30°
a = , B = °, C = °

2. Find the angles (to 1 decimal place) in the following triangles

(a) a = 2, b = 3, c = 4
A = °, B = °, C = °

(b) a = 1, b = 1, c = 1.5
A = °, B = °, C = °

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