函数名: acos 
 功  能: 反余弦函数 
 用  法: #include <math.h>
         double acos(double x); 
 程序例: 
 #include <stdio.h> 
 #include <math.h> 
int main(void) 
 { 
   double result; 
   double x = 0.5; 
  result = acos(x); 
   printf("The arc cosine of %lf is %lf\n", x, result); 
   return 0; 
 }