added area of three points

I am going to use the area of triangles when determining if the point is
inside of another square to determine containment.  This was just an
important step

triangle.go

@@ -0,0 +1,7 @@
+package rect
+
+import "math"
+
+func SizeTriangle(p1, p2, p3 Point) float64 {
+	return math.Abs((p1.X*p2.Y + p2.X*p3.Y + p3.X*p1.Y - p1.Y*p2.X - p2.Y*p3.X - p3.Y*p1.X) / 2)
+}

triangle_test.go

@@ -0,0 +1,25 @@
+package rect
+
+import "testing"
+
+func TestSizeTriangle(t *testing.T) {
+	var sizeTriangleTests = []struct {
+		pts      []Point
+		expected float64
+	}{
+		{[]Point{Point{-2, 3}, Point{-3, -1}, Point{3, -2}}, 12.5},
+		{[]Point{Point{0, 0}, Point{1, 1}, Point{0, 1}}, 0.5},
+		{[]Point{Point{10, 14}, Point{20, 15}, Point{12, 52}}, 189},
+	}
+	for _, rt := range sizeTriangleTests {
+		actual := SizeTriangle(rt.pts[0], rt.pts[1], rt.pts[2])
+		if actual != rt.expected {
+			t.Errorf(
+				"failed spiral:\n\texpected: %d\n\t  actual: %d",
+				rt.expected,
+				actual,
+			)
+		}
+
+	}
+}