```go
package main

import (
	"bufio"
	"fmt"
	"os"
)

func main() {
	in := bufio.NewReader(os.Stdin)
	out := bufio.NewWriter(os.Stdout)
	defer out.Flush()

	var t int
	fmt.Fscan(in, &t)

	for i := 0; i < t; i++ {
		var R, r, k float64
		fmt.Fscan(in, &R, &r, &k)

		diff := R - r
		ans := (R * r * diff) / (R*r + k*k*diff*diff)

		fmt.Fprintf(out, "%.10f\n", ans)
	}
}
```