Applications of Trigonometry
Choosing which rule or formula to use
- It is important to be able to decide which Rule or Formula to use to answer a question
- This table summarises the possibilities:
Using the cosine rules to find angles
- The Cosine Rule can be rearranged to give:
- When using the inverse cosine function (i.e. ) we can use this to find the size of angle :
- Make sure you can do this rearrangement, or remember this form of the formula
Using the sine rule to find angles
- If all we know are the lengths of and and the size of angle , there are two possible triangles that could be drawn
- one with side (and angle )
- the other with side (and angle )
- Using your calculator and the Sine Rule would only find you the possibility with angle
- You may need to subtract your answer from 180° to find the other angle
Examiner Tip
- In more involved exam questions, you may have to use both the Cosine Rule and the Sine Rule over several steps to find the final answer
- If your calculator gives you a ‘Maths ERROR’ message when trying to find an angle using the Cosine Rule, you probably subtracted things the wrong way around when you rearranged the formula
- The Sine Rule can also be written ‘flipped over’:
-
- This is more useful when we are using the rule to find angles
- When finding angles with the Sine Rule, use the info in the question to decide whether you have the acute angle case (ie the calculator value) or the obtuse angle case (ie, minus the calculator value)
- The Cosine Rule will never give you an ambiguous answer for an angle – as long as you put the right things into the calculator, the answer that comes out will be the correct angle
Worked example
In the following triangle:
The three side lengths are known and we want to find an angle so use the cosine rule.
Cosine Rule: , where is the angle opposite side
Rearrange to make the subject.
Use your calculator to find the value of .
Use the cos-1 button on your calculator to find the value of .
Substitute the values of into the sine rule.
Sine Rule: .
Rearrange the equation to find the value of
Subtract from 180° to find the value of obtuse angle