Jambmaths question:
If angle θ is 135o, evaluate cos θ
Option A:
$\frac{1}{2}$
Option B:
$\tfrac{\sqrt{2}}{2}$
Option C:
$\tfrac{-\sqrt{2}}{2}$
Option D:
$-\frac{1}{2}$
Jamb Maths Solution:
$\begin{align} & \cos {{135}^{0}}=\cos ({{180}^{0}}-{{45}^{o}}) \\ & \cos {{135}^{0}}=-\cos {{45}^{o}}\text{ (cosine is -ve in the 2nd quadrant)} \\ & \cos {{135}^{0}}=-\frac{1}{\sqrt{2}}=-\frac{1}{\sqrt{2}}\times \frac{\sqrt{2}}{\sqrt{2}} \\ & \cos {{135}^{0}}=-\frac{\sqrt{2}}{2} \end{align}$
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