What work, if any, have you done so far on this homework question?

Have you read the Help page for the plots:-animate command?

@Carl Love The use of f with two meanings in the question is confusing and poor. But it's not what prevented the OP's original approach from working. And it's not necessary to use the different name h in order to get it to work.

The OP's approach failed because he was making operators out of the equations z=blahblah returned by solve, and not accomodating that fact. One way his approach could work would be if he instead used only each RHS formula blahblah from the results returned by solve.

And that's how my Answer works; and it'd still work using f instead of f1 and f2. (Of course it'd be uglier and confusing if it were to so abuse the name f as the original question did.)

Tom's Answer uses a calling sequence of solve without sets, and so the results from solve are scalar expressions and not equations. Thus he avoids the problem with the OP's attempt. Tom's answer would work even if the operator he assigned to h were assigned to f. (But, there again, that'd be confusing.)

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?