math.h中的常量

类似于Matlab中经常用到的一些常量，C++里边也是有的。（经查源文件无意中看到）
写入如下代码：

#include<iostream>
#include<iomanip>
#define _USE_MATH_DEFINES
#include<math.h>
using namespace std;
int main()
{
system("color 3e");
int a=255;
cout<<setprecision(20);
cout<<"e          "<<M_E<<endl;
cout<<"log2(e)    "<<M_LOG2E<<endl;
cout<<"log10(e)   "<<M_LOG10E<<endl;
cout<<"ln(2)      "<<M_LN2<<endl;
cout<<"ln(10)     "<<M_LN10<<endl;
cout<<"pi         "<<M_PI<<endl;
cout<<"1/pi       "<<M_1_PI<<endl;
cout<<"2/pi       "<<M_2_PI<<endl;
cout<<"pi/2       "<<M_PI_2<<endl;
cout<<"pi/4       "<<M_PI_4<<endl;
cout<<"sqrt(2)    "<<M_SQRT2<<endl;
cout<<"1/sqrt(2)  "<<M_SQRT1_2<<endl;
cout<<"2/sqrt(pi) "<<M_2_SQRTPI<<endl;
cout<<"八进制：   "<<oct<<a<<endl;
cout<<"十进制：   "<<dec<<a<<endl;
cout<<"十六进制： "<<hex<<a<<endl;
cout<<"八进制：   "<<setbase(8)<<a<<endl;
cout<<"十进制：   "<<setbase(10)<<a<<endl;
cout<<"十六进制： "<<setbase(16)<<a<<endl;
return 0;
}