Surprising identities / equations 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.

$$\cos \left(20\right) \cos \left(40\right) \cos \left(80\right) = \frac{1}{8}$$
for angles in degrees.  This identity is interesting for its historical association with the teenage Richard Feynman.  From Genius by James Gleick:
"He and his friends traded mathematical tidbits like baseball cards.  If a boy named Morrie Jacobs told him that the cosine of 20 degrees multiplied by the cosine of 40 degrees multiplied by the cosine of 80 degrees equaled exactly one-eighth, he would remember that curiosity for the rest of his life, and he would remember that he was standing in Morrie's father's leather shop when he heard it."
