令一元二次方程判别式： b2 - 4ac为变量P

**当P > 0时：**

x1 = (-b+sqrt(P)) / 2a

x1 = (-b-sqrt(P)) / 2a

**当P = 0时：**

x1 = x2 = -b / 2a

**当P < 0时，一元二次方程无解当存在共轭复数：**

x1 = -b/2a + sqrt(-P)×i

x1 = -b/2a - sqrt(-P)×i

(注：i为虚数单位，如果高中数学好的话，应该知道)

**代码块**：

```java

import java.util.Scanner;

public class 一元二次方程 {

public static void main(String[] args) {

Scanner sc = new Scanner(System.in);

double a = sc.nextDouble();

double b = sc.nextDouble();

double c = sc.nextDouble();

double temp = b*b-4*a*c;

double x1=0, x2=0;

if(temp > 0){

x1 = (-b+Math.sqrt(temp))/(2*a);

x2 = (-b-Math.sqrt(temp))/(2*a);

}else if(temp == 0){

x1 = x2 = -b/(2*a);

}else{

double x11 = (-b)/(2*a);

double x12 = (Math.sqrt(-temp))/(2*a);

System.out.print

("x1="+String.format("%.3f", x11)+"+"+String.format("%.3fi", x12));

System.out.print

(" x2="+String.format("%.3f", x11)+"-"+String.format("%.3fi", x12));

return;

}

System.out.println

("x1="+String.format("%.3f",x1)+" x2="+String.format("%.3f",x2));

}

}

```

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