The `....:`

prompt is a continuation prompt, indicating that Sage is waiting for extra input, considering what you input so far is not complete. It might be because you have an open parenthesis or bracket that is not matched by a closing one, or because you have started a `def`

, `for`

, `if`

, `while`

, etc.

In the example you mention, I suspect you were missing a closing parenthesis.

In that case, Sage is waiting for more input, expecting a closing parenthesis at some point.

```
sage: f(x)=2.4/x^2*(1-e^(-x/50)
....:
```

If you close the parenthesis, hit return, and hit return one more time, your command is evaluated and you get back the usual Sage prompt.

```
sage: f(x)=2.4/x^2*(1-e^(-x/50)
....: )
....:
sage:
```

Now you can use the function you defined:

```
sage: f(x)
-2.40000000000000*(e^(-1/50*x) - 1)/x^2
```

If you enter a correct expression, which is not missing a closing parenthesis,
such as the one in your question, you should get the normal Sage prompt back immediately.
Using your example copy-pasted from your question doesn't give me a continuation prompt:

```
sage: f(x)=2.4/x^2*(1-e^(-x/50))
sage:
```

In case you are stuck with the `....:`

continuation prompt, check if some opening parenthesis or bracket is unmatched by a closing one. Or you can use Ctrl-C to abandon the current input and start afresh.