The sides of triangle ABC are as follows: AB = 3, AC = 2, BC = 4. Use the cosine rule, a^2=b^2+c^2-2bc cosA , to find the cosine of ∠BAC

Answer:

6) This is no longer in your BMAT specification.

If you do not know how to do it, then skip the question and move on! You cannot afford to waste time in the BMAT, especially in Section 2.

Draw the triangle out first. A visual representation always helps.

a = BC = 4
b = AC = 2
c = AB = 3
A = ∠BAC

a² = b² + c² – 2bc cosA
4² = 3² + 2² – 2 (2) (3) cosA
16 = 9 + 4 – 12 cosA
16 – 13 = – 12 cos A

³⁄₁₂ = – cos A
¹⁄₄ = – cosA
-¼ = cosA