So there are several trigonometric identities, some very well know, such as $\cos(x) = 1 - 2\sin^2(\frac{x}{2})$ and some more obscure like $\tan(\frac{\theta}{2} + \frac{\pi}{4}) = \sec(\theta)+\tan(\theta)$.
You can also find some logarithmic identities online
But looking for radical identities (stuff involving square roots), if I google I'm lead to politics and ethnicity (???)... I want to find some cool identities involving square roots that are always true for any $x>0$. I am aware they exist, I've seen some, but I don't remember them...
What are some cool obscure identities involving square roots?