start:java8

com/study/demo/test2_1.java

@@ -0,0 +1,14 @@
+package com.study.demo; 
+import java.util.Scanner;
+
+public class test4 {
+    public static void main(String[] args) {
+        Scanner sc = new Scanner(System.in);
+        double r1 = sc.nextDouble();
+        double r2 = sc.nextDouble();
+        sc.close();
+
+        double R = 1 / (1 / r1 + 1 / r2);
+        System.out.printf("%.2f\n", R);
+    }
+}