/ 一元二次方程
public class Main {
    public static void main(String[] args) {
        double a = 1.0;
        double b = 3.0;
        double c = -4.0;

        
// 求平方根可用 Math.sqrt():
        // System.out.println(Math.sqrt(2)); ==> 1.414
        // TODO:
        double d=b*2-4*a*c;
		double n=Math.sqrt(d);
		int y=(int)(n+0.5);
        System.out.println(Math.sqrt(d));
       
        double f=(-b+y)/2*a;
        double g=(-b-y)/2*a;
       
        double r1 = f;
        double r2 = g;


         System.out.println(r1);
        System.out.println(r2);
        System.out.println(r1 == 1 && r2 == -4 ? "测试通过" : "测试失败");
    }
}

4.69041575982343 

1.0 

-4.0 

测试通过