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Alexeev081 [22]
3 years ago
14

Find the exact value of Cos 240 degrees without a calculator. Show your work!

Mathematics
1 answer:
VMariaS [17]3 years ago
6 0

it equals <span>(−<span>12</span>)</span> because <span><span>cos<span>(<span>60∘</span>)</span></span>=<span>12</span></span>

Explanation:

The reference angle for <span>240∘</span> is <span>60∘</span> (since <span><span>240∘</span>=<span>180∘</span>+<span>60∘</span></span>)

<span>60∘</span> is an angle of one of the standard triangles with 
<span><span>cos<span>(<span>60∘</span>)</span></span>=<span>12</span></span>

<span>240∘</span> is in the 3rd quadrant so (either by CAST or noting that the "x-side" of the associate triangle is negative)
<span><span>cos<span>(<span>240∘</span>)</span></span>=−<span>cos<span>(<span>60∘</span>)</span></span></span>

<span><span>cos<span>(<span>240∘</span>)</span></span>=−<span><span>12</span></span></span>

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