Integrating an analog signal using Arduino Due
I think your main problem here is a fundamental lack of understanding about C++.  There's numerous basic errors in your code.  Let's take a look at them:

FeqIn3 = FeqIn1 - FeqIn2


All C++ statements must be terminated by a ; otherwise it doesn't know where one statement finishes and the next one begins.

area(1) = 0;
(and many others)


You have declared a bunch of float variables in the global scope.  You are then trying to use those float variables as functions, and on top of that you are then trying to assign a value to the return value of that function.  That makes absolutely no logical sense whatsoever.

I think you meant to be using arrays of float values.  Arrays are denoted by [ and ] not ( and ).  Further, they have to be defined as arrays, including the size of the array, not a single value.  Also in C++ arrays are zero based (i.e., they start at index 0 not index 1).

For example, area could be defined as:

float area[1024]; // 1024 floating point values in an array
// ...
area[0] = 0;


Next:

Recarea(n+1) = t(2)*(FeqIn3(n));


I don't even know what to make of this bit...  If I assume you should replace (...) with [...] for indices to arrays, it still makes no sense.  You're calculating a single value (FreqIn3 = FreqIn1 - FreqIn2) then using that as an array (FreqIn3[n]) which makes absolutely no sense to me at all.  Then you're using another value t which you are incrementing (incorrectly, more on that later), which again you are then trying to use as an array (t[2]).  Which is it - arrays or single values? And how do you increment an entire array?

Now the incrementing:

for (n = 1; n = t; t++)


n starts at the value 1.  While n is assigned the value t, increment t.  Now, does that sound logical?  And what is t? when that loop starts?  It has no defined value, so it could be anything at all.  And when does the loop end?  Well, since you're using float values for your counters (which is idiotic at best) you won't ever get any integer wraparound, so t will never equal 0 and consequently the result of assigning t to n will never be 0, so the loop will never end.

I think you need to go back to absolute basics and learn a bit more about syntax of C++, arrays, functions, loops, etc, before embarking on something complex like signal integration.