How do you use double angle formula to determine cos 120? Trigonometry Trigonometric Identities and Equations Double Angle Identities 1 Answer Alan P. 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# Answer link Related questions What are Double Angle Identities? How do you use a double angle identity to find the exact value of each expression? How do you use a double-angle identity to find the exact value of sin 120°? How do you use double angle identities to solve equations? How do you find all solutions for #sin 2x = cos x# for the interval #[0,2pi]#? How do you find all solutions for #4sinthetacostheta=sqrt(3)# for the interval #[0,2pi]#? How do you simplify #cosx(2sinx + cosx)-sin^2x#? If #tan x = 0.3#, then how do you find tan 2x? If #sin x= 5/3#, what is the sin 2x equal to? How do you prove #cos2A = 2cos^2 A - 1#? See all questions in Double Angle Identities Impact of this question 6995 views around the world You can reuse this answer Creative Commons License