How do you find the exact value for $cos 240$ ?

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Answer 1 · 2015-05-24T11:38:40

it equals $(-1/2)$ because $cos(60^@) = 1/2$

The reference angle for $240^@$ is $60^@$ (since $240^@ = 180^@ + 60^@$ )

$60^@$ is an angle of one of the standard triangles with
$cos(60^@) = 1/2$

$240^@$ is in the 3rd quadrant so (either by CAST or noting that the "x-side" of the associate triangle is negative)
$cos(240^@) = -cos(60^@)$

$cos(240^@) = -1/2$

How do you find the exact value for cos 240cos 240?