 # Find the value of \cos240°. Mary Hammonds 2021-12-26 Answered
Find the value of $\mathrm{cos}240°$.
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Reference angle for 240° is 60°: 240°=180°+60°
$\mathrm{cos}60°=\frac{1}{2}$
As $\mathrm{cos}240°=-\mathrm{cos}60°$, we have
$\mathrm{cos}240°=-\frac{1}{2}$
###### Not exactly what you’re looking for? Rita Miller
Write $\mathrm{cos}240°$ as $\mathrm{cos}\left(180°+60°\right)$
Using the summation identity:
$\mathrm{cos}\left(180°\right)\mathrm{cos}\left(60°\right)-\mathrm{sin}\left(180°\right)\mathrm{sin}\left(60°\right)$
Using trivial identity: $\mathrm{cos}180°=\left(-1\right),\mathrm{cos}60°=\frac{1}{2},\mathrm{sin}180°=0,\mathrm{sin}60°=\frac{\sqrt{3}}{2}$
$=\left(-1\right)×\frac{1}{2}-0×\frac{\sqrt{3}}{2}$
$=-\frac{1}{2}$
###### Not exactly what you’re looking for? karton