How do you evaluate #Cos (-285)#? Trigonometry Right Triangles Trigonometric Functions of Any Angle 1 Answer 冠廷 李. Jun 17, 2016 #(sqrt6-sqrt2)/4# Explanation: #cos(-285)=cos(-285+360)=cos(75)# #cos(75)=cos(30+45)# =#cos30cos45-sin30sin45=(sqrt6-sqrt2)/4# 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 2264 views around the world You can reuse this answer Creative Commons License