The scalar product a.b is defined as where is angle between a and b. From this definition it can also be shown that . You'll find both these versions on the formula sheet.
The main use of the scalar product is to calculate the angle .
Here's a worked example. Calculate the angle on the diagram below.
State the rule you are using for this question
If your answer at the substitution stage works out negative then the angle lies between 90° and 180°. For example, if then