三角函数tanpi怎样计算

为什么在Vc++6.0中 tan 3.1415926 = -0

在几何知识中tan(180) = 0，如下图所示;

 //THE PROGRAM IS TO TELL YOU HOW TO USE   
//THE MATH.H LIBRARY .   
//FILE MATH.CPP   

#define HUNDRED 100   
#include  <stdio.h>   
#include  <conio.h>   
#include   <math.h>   

int main(void)   
{   
    double CIRCLE,PI,VALUE;   
    printf("\n     -*-*- THE MATH FUNCTIOIN 'SIN()' & 'COS()'& 'TAN()'-*-*-\n");   
    printf("\nTHE DEGREE");   
    printf("   THE SIN() VALUE");   
    printf("     THE COS() VALUE ");   
    printf("     THE TAN() VALUE \n");   
    PI=3.1415926;   
    for (CIRCLE=0;CIRCLE<HUNDRED;CIRCLE+=10)   
    {   
        VALUE=CIRCLE*PI/180;   
        printf("%2d",(int)CIRCLE);   
        printf("         %6.4f",sin(VALUE));   
        printf("         %6.4f",cos(VALUE));   
        printf("                     %6.4f",tan(VALUE));   
        printf("\n");   
    }   
    getch();   
    return 0;   
}