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

1 Answer
Alan P.
May 24, 2015

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

Explanation:

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#

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