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WAEC Further Mathematics 2017 Paper

Question : 13

Total: 40

Express cos150° in surd form.

Solution:

Concept:

Use the related acute angle and the sign of cosine in the second quadrant.

Explanation:

150^\circ

lies in the second quadrant.

In the second quadrant, cosine is negative.

The reference angle is

180^\circ - 150^\circ = 30^\circ

So,

\cos 150^\circ = -\cos 30^\circ

We know

\cos 30^\circ = \frac{\sqrt{3}}{2}

Therefore,

\cos 150^\circ = -\frac{\sqrt{3}}{2}

Answer:

-\frac{\sqrt{3}}{2}

which is option B.

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