The question is :
If an operator $\hat{A}$ follows the property that $\hat{A}.\hat{A}=\mathbb{I}$ , where $\mathbb{I}$ is the identity operator, then prove that: $\exp(\theta \hat{A})=\cosh (\theta \mathbb{I}) + \sinh(\theta \hat{A})$, where $\theta$ is any scalar.
I can not prove this. Would you kindly help me to do this or provide me any link or useful reference, from where i can get the help.