Some small changes I made, along with additional functions.

I changed your system.pause( ) to cin.get(), it serves the same purpose.

I also added functions for Co-secant, Secant, and Cotangent. I'm not sure if those functions are needed, but I decided to be better safe than sorry.

Finally, I found an error where your error message would always be displayed, even if the user input was correct. That was fixed by using an else-if statement and not just an else.