using the Cosine rule to find the length of an unknown side
The Cosine Rule is used to find the length of an unknown side in a non right angled triangle. All you have to do is enter the values from the diagram into the formula. Learn how to enter all the values into your calculator in one go so you only have to hit the enter (or exe) button once.
Notice that the unknown side (x) is opposite the known angle 74º and the two known sides (b and c of the formula) are on either side of the 74º angle.
Enter the calculation like this and don't forget to round to two decimal places.
Your turn: find the unknown sides for following triangles.
ANSWERS: 1) 7.17m 2) 66.46m 3) 103.24m