Sum of $3$ triangles' angles: Geometric proof of $\tan^{-1}1+\tan^{-1}\frac12+\tan^{-1}\frac13$
89561
Find the sum of angles without trigonometry?
33905
Prove the following trigonometric identity: $\frac{\tan A+\sec A-1}{\tan A-\sec A+1}=\frac{1+\sin A}{\cos A}$
48824
Prove: $ \frac{1}{\sin 2x} + \frac{1}{\sin 4x } + \cdots + \frac{1 }{\sin 2^n x} = \cot x - \cot 2^n x $
83332
Trig sum: $\tan ^21^\circ+\tan ^22^\circ+\cdots+\tan^2 89^\circ = \text{?}$
103988
Prove that $\tan20^°\tan40^°\tan60^°\tan80^°=3$
86454
How to prove $\cos\left(\pi\over7\right)-\cos\left({2\pi}\over7\right)+\cos\left({3\pi}\over7\right)=\cos\left({\pi}\over3 \right)$
35244
Prove $\sin^2\theta + \cos^2\theta = 1$
90056
A synthetic geometrical proof required on a result regarding Isosceles triangles.
104427
Sines and cosines of angles in arithmetic progression
100895
Prove $1 + \cos 2C - \cos 2A - \cos 2B = 4\sin A\sin B\cos C$ for $A$, $B$, $C$ the angles of a triangle
134652
Prove that the perpendicular bisectors of the sides of a triangle meet at a point
60230
Prove that $\frac{1+\sin\theta+i\cos\theta}{1+\sin\theta-i\cos\theta}=\sin\theta+i\cos\theta$
46353
Proof that $\cos(\pi/3)=1/2$
73029
If a,b,c are sides of a triangle, prove: $ \sqrt{a+b-c} + \sqrt{b+c-a} + \sqrt{c+a-b} \le \sqrt{a} + \sqrt{b} + \sqrt{c} $
60673
To prove $\sum_{cyc}\frac{1}{a^3+b^3+abc} \le \frac{1}{abc}$
13297
Prove the following trigonometric identity without a calculator involved
146746
A formula for the sum of the triangular numbers?