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How do you use double angle formula to determine cos 120?

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Jun 5, 2015

Double Angle formula for Cosine (one version):
#color(white)("XXXX")##cos(2x) = 2cos^2(x)-1#

#cos(120^@)#
#color(white)("XXXX")##=cos(2*60^@)#

#color(white)("XXXX")##= 2* cos^2(60^@) -1#

#color(white)("XXXX")##= 2 * (1/2)^2 -1#

#color(white)("XXXX")## = -1/2#

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