Cosine Rule for Unknown Angle - Formula - Explanation - When to Use It - Example

We use the cosine rule to find an unknown angle in a triangle when we know all 3 side lengths and not a single interior angle.
In this tutorial we learn the formula to use for finding an unknown angle in a triangle, using the cosine rule.
We see how to read the formula, in particular we notice that the side length that's being subtracted on the numerator is always the side opposite the angle we're trying to find.
We then work through an example in which we see exactly how to use the formula.

Time Stamps

00:00 Introduction
00:19 Formula - Cosine Rule for Unknown Angle
01:27 Understanding How to Use the Formula
02:22 When to Use the Cosine Rule for an Unknown Angle
02:50 Worked Example