TITLE: Surprising identities / equations
QUESTION [339 upvotes]: What are some surprising equations/identities that you have seen, which you would not have expected?
This could be complex numbers, trigonometric identities, combinatorial results, algebraic results, etc.
I'd request to avoid 'standard' / well-known results like $ e^{i \pi} + 1 = 0$.
Please write a single identity (or group of identities) in each answer.
I found this list of Funny identities, in which there is some overlap.

REPLY [2 votes]: If $x^{x} = x+1$, then
$x^{x^2+x} = (x^2+x)^{x}$