Show the sizes of the other angles and the lengths of any lines that are known
Mark the angles or sides you have to calculate.
Consider whether you need to create right triangles by drawing extra lines. For example, divide an isosceles triangle into two congruent right triangles.
Decide whether you will need Pythagoras theorem, sine, cosine or tangent.
Check that your answer is reasonable. The hypotenuse is the longest side in a right triangle.
Example:
Calculate the value of cos θ in the following triangle.
Solution:
Use Pythagoras’ theorem to evaluate the length of PR.
The following video gives more examples of cosine problems.
