函数名: exp 
 功  能: 指数函数 
 用  法: #include <math.h>
         double exp(double x); 
 程序例: 
#include <stdio.h> 
 #include <math.h> 
int main(void) 
 { 
    double result; 
    double x = 4.0; 
   result = exp(x); 
    printf("'e' raised to the power \ 
    of %lf (e ^ %lf) = %lf\n", 
    x, x, result); 
   return 0; 
 }