Which version are you using?

Upload a worksheet so that we can see what you're obtaining. 

@shkarah I watched the video, as well as another by the same author, on plotting.

I would not recommend following that author's techniques for handling expressions with procedure calls which, to me, do not seem sensible and robust.

@Zeineb And why didn't you mark your Question as related to Maple 18, in the first place?

@shkarah Your definition of procedure f is not set up to handle the indexed, recursive call. And so your code has a nonsensical syntax, in my opinion.

You are using the name f for function calls in two different ways, and not set up by you to resolve the discrepancy.

Why don't you state in words precisely what you're trying to accomplish?

Upload and attach your actual document.

Where is x, in your definition of f?

@permanoon123 

In general you should not be trying to use ApproximateInt from the Student:-Calculus1 package to do general numeric integration. That command provides simple and basic approaches, provided mostly as a means of introductory instruction in the topic for undergraduate students.

Much more sophisticated and powerful techniques are available using the syntax evalf(Int(....)). See this comment for some description of the basic syntax. Or see the Help system.

Of course that's not going to help get you a finite result if your integral is divergent.

@permanoon123 So, why can't you be bothered to type in the integral in Maple yourself?

@permanoon123 Why do you want to work with the trapezoid rule?

What do you mean by stating that y=2, when y is used as a variable of integration?

What is f(y)?

@Carl Love I believe that the Equation Label needs to be shown to be used, and they are only shown alongside output.

I inserted a Label reference, and then went back and supressed output of the line that had that Label, by terminating the statement with a full colon. Upon re-execution of the whole sheet that inserted reference became "??" and the error "Invalid label reference" was emitted. This was in a Worksheet, in 1D input mode.

Moreover, only one Label gets shown if I have several statements in an Execution Group, each terminated with a semicolon. An inserted reference seems to get the last output.

Please provide a complete example.

Did you perhaps intend this as an optimization problem, to find values which made the equations close to zero?

Why set the data up in floating-point, and incur loss of precision up front?

Did you forget to attach something?

Which Maple version are you using?

@Carl Love There is some problematic aspect to that use of thisproc (but I'm not sure just what...), and style=pointline (Maple 2015?).

LogisticMapM1802.mw