# Doubling identity for cosh (1)

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## Theorem

The following formula holds: $$\cosh(2z)=2\cosh^2(z)-1,$$ where $\cosh$ denotes hyperbolic cosine.

## Proof

## References

- 1964: Milton Abramowitz and Irene A. Stegun:
*Handbook of mathematical functions*... (previous) ... (next): $4.5.32$