How do you prove costhetacottheta=costheta? Trigonometry Trigonometric Identities and Equations Proving Identities 1 Answer sente Oct 24, 2016 The identity is not true in general Explanation: Consider theta = pi/3. Then cos(theta) = 1/2, and cot(theta) = cos(theta)/sin(theta) = (1/2)/(sqrt(3)/2) =sqrt(3)/3 So cos(theta)cot(theta) = 1/2(sqrt(3)/3) = sqrt(3)/6 != 1/2 = cos(theta) Answer link Related questions What does it mean to prove a trigonometric identity? How do you prove \csc \theta \times \tan \theta = \sec \theta? How do you prove (1-\cos^2 x)(1+\cot^2 x) = 1? How do you show that 2 \sin x \cos x = \sin 2x? is true for (5pi)/6? How do you prove that sec xcot x = csc x? How do you prove that cos 2x(1 + tan 2x) = 1? How do you prove that (2sinx)/[secx(cos4x-sin4x)]=tan2x? How do you verify the identity: -cotx =(sin3x+sinx)/(cos3x-cosx)? How do you prove that (tanx+cosx)/(1+sinx)=secx? How do you prove the identity (sinx - cosx)/(sinx + cosx) = (2sin^2x-1)/(1+2sinxcosx)? See all questions in Proving Identities Impact of this question 1184 views around the world You can reuse this answer Creative Commons License