The function f(x) does the following:
1. Capitalizes the first letter of the input string
2. Appends the letter 'O' to the end of the input string.

[CODE]:
```python
def approx_f(x: str) -> str:
    return x.capitalize() + 'O'
```
To make sure this function behaves similarly to the original f(x), please test it on a few more inputs:
COMMAND: PYTHON(./temp/function.py "python" "successful" "!test")