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mathematicsprinted 107 · PDF 108
165 pagesrevision 46935
printed 107 · PDF 108
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107
In the above example, 2 sides and an angle are given and angle θ needs to
be found. It is apparent that the angles and sides are opposite each other so
the sine rule can be applied. θ can now be found.
Cosine Rule
The cosine rule, again, is different from the cos trigonometric identity but it
incorporates it into its formula. This is also for non-right angled triangles. You
will need to identify whether the sine or cosine rule needs to be applied
depending on the known values in the question and what the question is
asking for.
𝐴2 = 𝐵2 + 𝐶2 −2𝐵𝐶⋅𝑐𝑜𝑠(𝑎)
𝐴, 𝐵 and 𝐶 are the sides of the triangle and angle 𝑎 is the angle opposite to
side 𝐴. This is similar to the sine rule where the angle 𝑎 and side 𝐴 need to be
opposite each other. Sides 𝐵 and 𝐶 are the remaining 2 sides.