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Quote: Originally Posted by GeoffP Q. Prove Cot^2t-Cos^2t=Cot^2t . Cos^2t I have changed to sines/Cosines but seem to end up going round in circles. I have also tried working on both sides. I think i need a combination of Cos^2t= 1- Sin^2t and Cot^2t = Cos^2t / Sin^2 t. Any assistance appreciated. Cheers Note that $\displaystyle \cot^2t-\cos^2t=\frac{\cos^2t}{\sin^2t}-\cos^2t=\left(\frac{1}{\sin^2t}-1\right)\cos^2t=\left(\csc^2t-1\right)\cos^2t$ Apply another identity, and you're done.