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 :
This form of the formula is not on your exam formula sheet, so make sure you can do the rearrangement yourself!
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 angle you need
Exam 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:a) Find the size of angle ABC.
b) Given that angle ACB is obtuse, use the Sine Rule and your answer from (a) to find the size of angle ABC.