understanding #define with C

C Syntax
1.#define A B
   #define seneca 100
   #define PI 3.1415926
   #define   A(x) x
-------------------------------------------------
example:
#define A (a+b)

　　main(){

　　int x,y;

　　printf("please input a number: ");

　　scanf("%d",&y);

　　x=A*A;

　　printf("x=%d\n",x);

　　}

2.
#ifndefine __headerfileABC__
#define __headerfileABC__
......
#endif

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hello, everyone! this is my blog which is to share my opinions about using C and C++.