# Derivative of a function

Hi, I've got this two functions:

```
psi=function('psi',t)
psi_d=function('psi_d',t)
psi=(sin(delta(t))*v(t))/(l*cos(delta(t)))
psi_d=diff(psi)
```

but it says *No differentiation variable specified* how can I derivate psi? Please someone help me it's driving me crazy since yesterday!

Thank you very much

By the way, in general the first thing you should do when you encounter an error like that is to make sure you didn't accidentally make a mistake in the syntax. This help can be gotten by typing

`diff?`

and it should tell you then that you need the variable of differentiation, as pointed out below.