SOLUTION: What is angle CAB using the cosine rule when AB = 12cm, AC = 16 cm and BC = 20 cm? Please show your working out.
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Question 729053: What is angle CAB using the cosine rule when AB = 12cm, AC = 16 cm and BC = 20 cm? Please show your working out.
Answer by Simnepi(216) (Show Source): You can put this solution on YOUR website!
using the cosine rule is a waste of time!
if you draw the triangle, label it and examine the side lengths. They all can be divided by 4 giving a 3, 4, 5 triangle and is therefore a right angled triangle. so the angle CAB is 90 degrees.
HOWEVER if the cosine rule MUST be used........
the cosine rule is
rearranging gives
substituting values gives us
evaluating gives
i.e. Cos(A)=0
therefore A = 90 degrees
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