Intuitive proof of the sin and cos addition formulas?
34049
How can I understand and prove the "sum and difference formulas" in trigonometry?
43605
If $\sin(a)\sin(b)\sin(c)+\cos(a)\cos(b)=1$ then find the value of $\sin(c)$
73538
solving $\cos z + \sin z = i$
51979
Is there a geometrical method to prove $x<\frac{\sin x +\tan x}{2}$?
127380
Use vectors to prove $\cos(\beta - \alpha) = \cos \alpha \cos\beta + \sin\alpha \sin\beta$
94283
Different ways finding the derivative of $\sin$ and $\cos$.
149686
Combinatorics and trigonometry identity
17188
Simplify $\cos(n \pi)$ and $\sin(n \pi)$ when n is an integer
72157
Elementary proof of tangent half angle formula
9986
Prove $ \sin(A+B)\sin(A-B)=\sin^2A-\sin^2B $
56934
How do we prove $\cos(\pi/5) - \cos(2\pi/5) = 0.5$ ?.
58633
Proof of the formula $1+x+x^2+x^3+ \cdots +x^n =\frac{x^{n+1}-1}{x-1}$
116270
Functional equations leading to sine and cosine
21941
Proving: $x = a_0 + \sum_{n=1}^{\infty}(a_n \cos nx + b_n \sin nx)$
57750
Solve identity: $\frac {\sin x}{1-\cos x} = \frac {1+\cos x}{\sin x}$
104427
Sines and cosines of angles in arithmetic progression
84420
When is $\sin(x) = \cos(x)$?