The most obvious errors:
You are not using the classes you have created. Once you have defined area class, you have to
create an instance of area class and store it in a variable to access its methods easily.
int main()
{
area myArea;
area.rectangle();
area.circle();
}
As said in code comments, the second issue is that you are trying to call rectangle() and circle() functions without providing them with their parameters. The function call must match function signature, otherwise compiler will issue an error.
Maybe a quick refresh on c++ 101, and its usage of classes, could suit you?
C++ Classes and Objects[
^]
You will also meet an issue with arr2, since you will store in an integer the result of a double operation; your results for the circle will be truncated values (values without their decimal parts). You should double type for arr2; and probably for arr1 as well, as a matter of consistency.
Last, you should try to give meaningful name to your variables:
arr2 = pi*r*r;
circleArea = pi*r*r;
This makes things a lot easier for you and for anyone who wants to read the code.