程序代码：

#include <iostream>
#include <cmath>
#include <numbers>

int main( void )
{
    double a = sqrt(3);
    double b = 3;
    double c = sqrt(12);

    double A = acos( (b*b+c*c-a*a)/(2*b*c) ) * std::numbers::inv_pi * 180;
    double B = acos( (c*c+a*a-b*b)/(2*c*a) ) * std::numbers::inv_pi * 180;
    double C = acos( (a*a+b*b-c*c)/(2*a*b) ) * std::numbers::inv_pi * 180;

    std::cout << A << std::endl;
    std::cout << B << std::endl;
    std::cout << C << std::endl;
}