从键盘输入一个角度值，求出该角度的正弦值、余弦值和正切值。
系统函数中提供了求正弦值、余弦值和正切值的函数：sin()、cos()、tan()，函数
的说明在头文件 cmath 中。
#include <iostream>
#include <cmath>
using namespace std;
const double PI = 3.14159265358979;
int main()
{
double angle;
cout << "Please enter an angle: ";
cin >> angle; //输入角度值
double radian = angle * PI / 180; //转为弧度
cout << "sin(" << angle << ") = " << sin(radian) <<endl;
cout << "cos(" << angle << ") = " << cos(radian) <<endl;
cout << "tan(" << angle << ") = " << tan(radian) <<endl;
return 0;
}