Reciprocal of Hyperbolic Cosine Minus One
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Theorem
- $\dfrac 1 {\cosh x - 1} = \dfrac 1 2 \csch^2 \dfrac x 2$
Proof
\(\ds \cosh x\) | \(=\) | \(\ds 1 + 2 \sinh^2 \frac x 2\) | Double Angle Formula for Hyperbolic Cosine: Corollary $2$ | |||||||||||
\(\ds \leadstoandfrom \ \ \) | \(\ds \cosh x - 1\) | \(=\) | \(\ds 2 \sinh^2 \frac x 2\) | subtracting $1$ from both sides | ||||||||||
\(\ds \leadstoandfrom \ \ \) | \(\ds \frac 1 {\cosh x - 1}\) | \(=\) | \(\ds \frac 1 2 \frac 1 {\sinh^2 \frac x 2}\) | taking the reciprocal of both sides | ||||||||||
\(\ds \) | \(=\) | \(\ds \frac 1 2 \csch^2 \frac x 2\) | Definition 2 of Hyperbolic Cosecant |
$\blacksquare$