The diagram shows the positions of three towns, A, B and C. AB is 73 km, BC is 62 km and AC is 56 km. A is due North of C.

  1. (a)

    Calculate the bearing of B from A.

    Answer ...

  2. (b)

    Calculate the area of triangle ABC.

    Answer ... km^2

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Worked solution

20(a)

1. Identify the angle needed in the triangle

Since AA is due North of CC, the line ACAC is a north-south line.

Find ∠BAC\angle BAC, the angle between ABAB and ACAC.

2. Use the cosine rule to calculate $\angle BAC$

By the cosine rule,
BC2=AB2+AC2−2(AB)(AC)cos⁡∠BACBC^2=AB^2+AC^2-2(AB)(AC)\cos \angle BAC
622=732+562−2(73)(56)cos⁡∠BAC62^2=73^2+56^2-2(73)(56)\cos \angle BAC
cos⁡∠BAC=732+562−6222(73)(56)\cos \angle BAC=\frac{73^2+56^2-62^2}{2(73)(56)}
cos⁡∠BAC=46218176\cos \angle BAC=\frac{4621}{8176}

3. Evaluate the angle and convert it to a bearing

∠BAC=cos⁡−1(46218176)\angle BAC=\cos^{-1}\left(\frac{4621}{8176}\right)
∠BAC≈55.6∘\angle BAC\approx55.6^\circ

From AA, the direction of CC is due South, which has bearing 180∘180^\circ.

Since BB is to the east of ACAC as shown in the diagram,
bearing of B from A=180∘−55.6∘=124.4∘\text{bearing of }B\text{ from }A=180^\circ-55.6^\circ=124.4^\circ

To the nearest degree, the bearing is
124∘.124^\circ.

Answer

124∘124^\circ

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