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T. Amdeberhan
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One trig "survives" a binomial summation: why?

I've seen many trigonometric identities but here is one that I encountered for which I did not find a reference.

In case you wonder where this came from, I was investigating certain $q$-series in this paper on Eulerian polynomials.

Question. Is this true? Can you provide a reference or a proof? $$\sum_{k=-n}^n\frac{2k+1}{n+k+1}\binom{2n}{n-k}\sec((2k+1)\theta)=\sec((2n+1)\theta).$$

T. Amdeberhan
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