Si sabenos que :
<img src="https://tex.z-dn.net/?f=tanh%28x%29%3D%20%5Cfrac%7Bsenh%28x%29%7D%7Bcosh%28x%29%7D%20" />
Reemplazamos :
<img src="https://tex.z-dn.net/?f=%5Cfrac%7B%20%5Cfrac%7Bsenh%28x%29%7D%7Bcosh%28x%29%7D%7D%7B1-%20%5Cfrac%7Bsenh%5E%7B2%7D%20%28x%29%7D%7Bcosh%5E%7B2%7D%20%28x%29%7D%7D%3Dsenh%5E%7B2%7D%20%28x%29" />
<img src="https://tex.z-dn.net/?f=%20%5Cfrac%7B%20%5Cfrac%7Bsenh%28x%29%7D%7Bcosh%28x%29%7D%7D%7B%20%5Cfrac%7Bcosh%5E%7B2%7D%28x%29%20-%20senh%5E%7B2%7D%28x%29%20%20%7D%7Bcosh%5E%7B2%7D%28x%29%7D%20%7D%20%3Dsenh%5E%7B2%7D%20%28x%29" />
Luego por formula : <img src="https://tex.z-dn.net/?f=%7Bcosh%5E%7B2%7D%28x%29%20-%20senh%5E%7B2%7D%28x%29%20%7D%3D1" /> , y reemplazamos :
<img src="https://tex.z-dn.net/?f=%5Cfrac%7B%20%5Cfrac%7Bsenh%28x%29%7D%7Bcosh%28x%29%7D%7D%7B%20%5Cfrac%7B1%7D%7Bcosh%5E%7B2%7D%28x%29%7D%20%7D%20%3Dsenh%5E%7B2%7D%20%28x%29" />
Multiplicamos :
<img src="https://tex.z-dn.net/?f=%5Cfrac%7Bsenh%28x%29cosh%5E%7B2%7D%20%28x%29%7D%7Bcosh%28x%29%7D%20%3Dsenh%5E%7B2%7D%20%28x%29" />
simplificamos :
<img src="https://tex.z-dn.net/?f=%7Bsenh%28x%29%7D%7Bcosh%28x%29%7D%20%3Dsenh%5E%7B2%7D%20%28x%29" />
Aqui para que se cumpla la igualdad tenemos que hacer que :
<img src="https://tex.z-dn.net/?f=senh%28x%29%3Dcosh%28x%29" />
Donde reemplazando tenemos :
<img src="https://tex.z-dn.net/?f=%7Bsenh%28x%29senh%28x%29%7D%20%3Dsenh%5E%7B2%7D%20%28x%29" />
Donde se cumple la identidad :
<img src="https://tex.z-dn.net/?f=%7Bsenh%5E%7B2%7D%28x%29%7D%20%3Dsenh%5E%7B2%7D%20%28x%29" />.