add: numbers and strings exercises

08-numbers-and-strings/01-numbers/exercises/08-sphere-area/main.go

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+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import (
+	"fmt"
+)
+
+// ---------------------------------------------------------
+// EXERCISE: Sphere Area
+//
+//  1. Get the radius from the command-line
+//  2. Convert it to a float64
+//  3. Calculate the surface area of a sphere
+//
+// SPHERE SURFACE AREA FORMULA
+//  area = 4πr²
+//  https://en.wikipedia.org/wiki/Sphere#Surface_area
+//
+// RESTRICTION
+//  Use `math.Pow` to calculate the area
+//  Read its documentation to see how it works.
+//  https://golang.org/pkg/math/#Pow
+//
+// EXPECTED OUTPUT
+//  1256.64
+// ---------------------------------------------------------
+
+func main() {
+	var radius, area float64
+
+	// ADD YOUR CODE HERE
+	// ...
+
+	// DO NOT TOUCH THIS
+	fmt.Printf("radius: %g -> area: %.2f\n", radius, area)
+}

08-numbers-and-strings/01-numbers/exercises/08-sphere-area/solution/main.go

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+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import (
+	"fmt"
+	"math"
+	"os"
+	"strconv"
+)
+
+func main() {
+	var radius, area float64
+
+	radius, _ = strconv.ParseFloat(os.Args[1], 64)
+
+	area = 4 * math.Pi * math.Pow(radius, 2)
+
+	fmt.Printf("radius: %g -> area: %.2f\n",
+		radius, area)
+}