why does there exist a unitary matrix U for every symmetric matrix A such that $A=UDU^{\dagger}$ where D is a diagonal matrix?

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I am not able to see why this is true but it looks a bit like the Jacobian rule? Any explanation on why this is true would be much appreciate!

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This is the spectral theorem. See the normal matrix section of the wiki page

https://en.m.wikipedia.org/wiki/Spectral_theorem