Find the value of #cos2theta# for #theta=pi/4#? Trigonometry Right Triangles Trigonometric Functions of Any Angle 1 Answer Shwetank Mauria Nov 13, 2016 #cos2theta=0# Explanation: #cos2theta=cos(theta+theta)# = #costhetaxxcostheta-sinthetaxxsintheta# as for #theta=pi/4#, #costheta=sintheta=1/sqrt2# #cos2theta=costhetaxxcostheta-sinthetaxxsintheta# = #1/sqrt2xx1/sqrt2-1/sqrt2xx1/sqrt2# = #1/2-1/2=0# 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 3400 views around the world You can reuse this answer Creative Commons License