The best thing to use a function is first to declare it before you even get into the main(), which is the case where all the examples in the tutorial book do. This is in my case.

However, here is the question. I am quite confused with the name or variable of the function as well as the argument_list.

For example, in the book here is the example of it.

Code:
// Function must be declared before being used.

(I have omitted some of the lines or codes, and this is how it is declared)

int triangle(int num); --> this is the declaration of the function and int num is the argument 

main(){

(this area is omitted)

}

// Triangle number function

int triangle(int n){ --> This is how the function is being defined

(this area is omitted)

}
My question is whether the argument (int num) in the declaration of the function has to be the same as the argument (int n) in the defined function? Or can be they different or I could use any name for them? Because sometimes when I browsed through the book, in some examples, they are just the same but sometimes a bit different and as I am new to function, it would be quite a bit of confusing to me. I am being baffled.