opengl求圆坐标glVertex2f(R*cos(2*Pi/n*i), R*sin(2*Pi/n*i))中2*Pi/n*i的原理是什么，哪位高手能讲解上

#include <windows.h>
#include <glut.h>
#include <glext.h>
#include <time.h>
#include<math.h>
const int n = 1000;
const GLfloat R = 0.5f;
const GLfloat Pi = 3.1415926536f;

void DrawCircle()
{
    int i;
    glBegin(GL_LINE_LOOP);
    for(i=0; i<n; ++i)
        [color=#FF0000]glVertex2f(R*cos(2*Pi/n*i), R*sin(2*Pi/n*i));[/color]
    glEnd();

    glBegin(GL_LINES);
        glColor3f(1, 0, 0);
        glVertex3f(0, 0, 0);
        glVertex3f(1.2*R, 0, 0);

        glColor3f(0, 1, 0);
        glVertex3f(0, 0, 0);
        glVertex3f(0, 1.2*R, 0);
    glEnd();
    glFlush();
}
void myDisplay()
{
    glClear(GL_COLOR_BUFFER_BIT);
    DrawCircle();
}

int main(int argc, char *argv[])
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB | GLUT_DEPTH);
    glutInitWindowPosition(100, 100);
    glutInitWindowSize(400, 400);
    glutCreateWindow("Draw a circle");
    glutDisplayFunc(myDisplay);
    glutMainLoop();
    return 0;
}