shapes: add Triangle area test

shapes/shapes.go

@@ -1,6 +1,8 @@
 package shapes
 
-import "math"
+import (
+	"math"
+)
 
 type Shape interface {
 	Area() float64
@@ -11,10 +13,6 @@ type Rectangle struct {
 	Height float64
 }
 
-type Circle struct {
-	Radius float64
-}
-
 func (r Rectangle) Perimeter() float64 {
 	return 2 * (r.Width + r.Height)
 }
@@ -23,6 +21,19 @@ func (r Rectangle) Area() float64 {
 	return r.Width * r.Height
 }
 
+type Circle struct {
+	Radius float64
+}
+
 func (c Circle) Area() float64 {
 	return c.Radius * c.Radius * math.Pi
 }
+
+type Triangle struct {
+	Width  float64
+	Height float64
+}
+
+func (t Triangle) Area() float64 {
+	return t.Width * t.Height * 0.5
+}

shapes/shapes_test.go

@@ -19,6 +19,7 @@ func TestArea(t *testing.T) {
 	}{
 		{Rectangle{30.5, 20.5}, 625.25},
 		{Circle{10}, 314.1592653589793},
+		{Triangle{12, 6}, 36.0},
 	}
 
 	for _, test := range areaTests {