// 求平方根可用 Math.sqrt():

 // System.out.println(Math.sqrt(2)); ==> 1.414

 // TODO:

double r1 = 0;

 double r2 = 0;

r1 = (Math.sqrt(b * b - 4 * a * c) - b) / (2 * a);
r2 = (-b - Math.sqrt(b * b - 4 * a * c))  / (2 * a);