How do you use the double-angle formula to rewrite the expression: #(1/3)cos^2x-(1/6)#? Trigonometry Trigonometric Identities and Equations Double Angle Identities 1 Answer Nghi N. Apr 29, 2015 Use the trig identity: #cos 2a = 2. cos^2 (a) - 1#. #y = 1/3. cos^2 (x/3) - (1/6) = 1/6. (2cos^2 (x/3) - 1) = 1/6 cos ((2x)/3)# 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 7862 views around the world You can reuse this answer Creative Commons License