add: numbers and strings exercises

08-numbers-and-strings/01-numbers/exercises/07-circle-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: Circle Area
+//
+//  Calculate the area of a circle from the given radius
+//
+// CIRCLE AREA FORMULA
+//  area = πr²
+//  https://en.wikipedia.org/wiki/Area#Circles
+//
+// HINT
+//  For PI you can use `math.Pi`
+//
+// EXPECTED OUTPUT
+//  314.1592653589793
+//
+// BONUS EXERCISE!
+//   1. Print the area as 314.16
+//   2. To do that you need to use the correct Printf verb :)
+//      Instead of `%g` verb below.
+// ---------------------------------------------------------
+
+func main() {
+	var (
+		radius = 10.
+		area   float64
+	)
+
+	// ADD YOUR CODE HERE
+	// ...
+
+	// DO NOT TOUCH THIS
+	fmt.Printf("radius: %g -> area: %g\n", radius, area)
+}

08-numbers-and-strings/01-numbers/exercises/07-circle-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"
+)
+
+func main() {
+	var (
+		radius = 10.
+		area   float64
+	)
+
+	area = math.Pi * radius * radius
+
+	fmt.Printf("radius: %g -> area: %.2f\n",
+		radius, area)
+
+	// ALTERNATIVE:
+	// math.Pow calculates the power of a float number
+	// area = math.Pi * math.Pow(radius, 2)
+}