Find the value of $c o s 120^{o}$

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Solution

Find the value of $c o s 120^{o}$ using formula for allied angles
$\cos 120 ° = \cos (90 + 30) °$
$= - \sin 30 ° = - \frac{1}{2}$ $. . . [∵ \cos (90 + θ) = - \sin θ]$
Hence the value of $c o s 120^{o}$ is $- \frac{1}{2}$

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