Why is $\cos(\arctan(3x)) = \frac{1}{\sqrt{1 + 9x^2}}?$
115128
How do I show this $\cos^2(\arctan(x))=\frac{1}{1+x^2}$
23397
Why it's true? $\arcsin(x) +\arccos(x) = \frac{\pi}{2}$
40556
Why is $\sin(\alpha+\beta)=\sin\alpha\cos\beta+\sin\beta\cos\alpha$
36023
Showing that $ \sum_{n=1}^{\infty} \arctan \left( \frac{2}{n^2} \right) =\frac{3\pi}{4}$
48956
Simplify $\arctan (\frac{1}{2}\tan (2A)) + \arctan (\cot (A)) + \arctan (\cot ^{3}(A)) $
122920
Why is $y + x = 3$ not the same as $y^2 + x^2 = 9$
33905
Prove the following trigonometric identity: $\frac{\tan A+\sec A-1}{\tan A-\sec A+1}=\frac{1+\sin A}{\cos A}$
117393
Behavior of ArcTan when two zeros are given as arguments
82586
Why is cos -x equal to cos x?
104453
A question about the arctangent addition formula.
142365
Use of substitution into the integral $\int_0^2 \frac{\arctan x}{x^2+2x+2}dx$
21855
Prove $\sum_{n=1}^{\infty}\arctan{\left(\frac{1}{n^2+1}\right)}=\arctan{\left(\tan\left(\pi\sqrt{\frac{\sqrt{2}-1}{2}}\right)\cdots\right)}$
121057
Find a formula relating $\operatorname{arcsin}(x)$ and $\operatorname{arccos}⁡(x)$
90226
Proving trigonometric equation $\cos(36^\circ) - \cos(72^\circ) = 1/2$
88432
A problem on range of a trigonometric function: what is the range of $\frac{\sqrt{3}\sin x}{2+\cos x}$?
46353
Proof that $\cos(\pi/3)=1/2$
31852
An elegant way to solve $\frac {\sqrt3 - 1}{\sin x} + \frac {\sqrt3 + 1}{\cos x} = 4\sqrt2 $