WAEC 2020 · Paper 1 · Q3

Trigonometry

If $\cos x = -0.7133$ , find the values of $x$ between $0^\circ$ and $360^\circ$ .

Worked solution (try it first)

Find the acute angle first: $\cos^{-1}(0.7133) = 44.5^\circ$ .
Cosine is negative in the second and third quadrants.
Second quadrant: $180^\circ - 44.5^\circ = 135.5^\circ$ .
Third quadrant: $180^\circ + 44.5^\circ = 224.5^\circ$ .
So $x = 135.5^\circ$ or $224.5^\circ$ , option D.

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