How do I find the value of cos 9π/4? Trigonometry Right Triangles Trigonometric Functions of Any Angle 1 Answer Alan P. Oct 24, 2015 #cos((9pi)/4) = sqrt(2)/2# Explanation: #(9pi)/4 = 2pi +pi/4# So cos((9pi)/4) = cos(pi/4)# #pi/4# is one of the standard triangle angles and #cos(pi/4) = sqrt(2)/2 (or 1/sqrt(2))# 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 21074 views around the world You can reuse this answer Creative Commons License