How do you simplify #cos^2((2pi)/9) - sin^2((2pi)/9)#? Trigonometry Right Triangles Trigonometric Functions of Any Angle 1 Answer A. S. Adikesavan Apr 12, 2016 #cos(80^o)=0.17365#, nearly Explanation: #cos^2A-sin^2A=cos 2A# Here, #A = 2pi/9 radian=2X180/9 = 40^o#. Answer is cos 2A = #cos(80^o)=0.17365#, nearly Answer link Related questions How do you find the trigonometric functions of any angle? What is the reference angle? How do you use the ordered pairs on a unit circle to evaluate a trigonometric function of any angle? What is the reference angle for #140^\circ#? How do you find the value of #cot 300^@#? What is the value of #sin -45^@#? How do you find the trigonometric functions of values that are greater than #360^@#? How do you use the reference angles to find #sin210cos330-tan 135#? How do you know if #sin 30 = sin 150#? How do you show that #(costheta)(sectheta) = 1# if #theta=pi/4#? See all questions in Trigonometric Functions of Any Angle Impact of this question 2715 views around the world You can reuse this answer Creative Commons License