Why am I getting output as -41. Assume all header files are there?

Question

#define multiply(x) (x*x)

int c=multiply(4-9);
cout<<c;

Answer 1

This is because the preprocessor just does a literal substitution.

So int c=multiply(4-9); is just substituted (during pre-processing stage) with:
int c=(4-9*4-9);

Because of algebraic rules of precedence the multiplication is calculated first:
int c=(4-36-9);
The answer is then -41.

If you want that to work correctly you can do this:
#define multiply(x) ((x)*(x))

BTW, if you turn your preprocessor function into a normal function it would work correctly:
int multiply(int x){return x*x;}

The reason it works here is because the expression is resolved first, and then the result of the expression is passed as argument to the function. So the x*x step in the normal function would be doing -5*-5, whilst the preprocessor version is doing (4-9)*(4-9).