diff --git a/src/cmd/math/main.go b/src/cmd/math/main.go
new file mode 100644
index 0000000..79eebde
--- /dev/null
+++ b/src/cmd/math/main.go
@@ -0,0 +1,49 @@
+package main
+
+import (
+	"github.com/surdeus/gox/src/gx"
+	"fmt"
+)
+
+func main() {
+	lines := []gx.Line{
+		gx.LineSegment{
+			gx.Point{0, 1},
+			gx.Point{1, 2},
+		}.Line(),
+		gx.LineSegment{
+			gx.Point{0, 5},
+			gx.Point{1, 2},
+		}.Line(),
+		gx.LineSegment{
+			gx.Point{-1, -1},
+			gx.Point{1, 50},
+		}.Line(),
+	}
+	
+	for _, l := range lines {
+		fmt.Println(l.Equation())
+	}
+	/*t := gx.Triangle{
+		gx.Point{0, 0},
+		gx.Point{0, 100},
+		gx.Point{100, 0},
+	}
+	
+	points := []gx.Point{
+		gx.Point{},
+		gx.Point{.1, .1},
+		gx.Point{-1, -1},
+		gx.Point{1, 1},
+		gx.Point{101, 1},
+		gx.Point{100, 1},
+		gx.Point{50, 1},
+	}
+	
+	for _, p := range points {
+		fmt.Println(p, t.PointIsIn(p))
+	}*/
+}
+
+
+
diff --git a/src/gx/line-segment.go b/src/gx/line-segment.go
deleted file mode 100644
index 79c8e84..0000000
--- a/src/gx/line-segment.go
+++ /dev/null
@@ -1,17 +0,0 @@
-package gx
-
-import (
-	"math"
-)
-
-type LineSegment [2]Point
-
-func (ls LineSegment) LenSqr() Float {
-	return Sqr(ls[0].X - ls[1].X) +
-		Sqr(ls[0].Y - ls[1].Y)
-}
-
-func (ls LineSegment) Len() Float {
-	return Sqrt(ls.LenSqr())
-}
-
diff --git a/src/gx/line.go b/src/gx/line.go
new file mode 100644
index 0000000..5d526d3
--- /dev/null
+++ b/src/gx/line.go
@@ -0,0 +1,43 @@
+package gx
+
+import (
+	"math"
+)
+
+// The type represents mathematical equation of line.
+type LineEquation struct {
+	K, C Float
+}
+
+// The type represents a line segment.
+type LineSegment [2]Point
+
+// The type represents a line.
+type Line LineSegment
+
+// Get square of length of line segment.
+func (ls LineSegment) LenSqr() Float {
+	return Sqr(ls[0].X - ls[1].X) +
+		Sqr(ls[0].Y - ls[1].Y)
+}
+
+// Get length of the line segment.
+func (ls LineSegment) Len() Float {
+	return math.Sqrt(ls.LenSqr())
+}
+
+// Returns corresponding to the line segment line.
+func (ls LineSegment) Line() Line {
+	return Line(ls)
+}
+
+func (l Line) Equation() LineEquation {
+	p0 := l[0]
+	p1 := l[1]
+	
+	k := (p0.Y - p1.Y) / (p0.X - p1.X)
+	c := p0.Y - p0.X*k 
+	
+	return LineEquation{k, c}
+}
+
diff --git a/src/gx/math.go b/src/gx/math.go
index 3578a0d..c80ea15 100644
--- a/src/gx/math.go
+++ b/src/gx/math.go
@@ -1,5 +1,9 @@
 package gx
 
+import (
+	"math"
+)
+
 // The type is used in all Engine interactions
 // where you need floating values.
 type Float = float64
diff --git a/src/gx/triangle.go b/src/gx/triangle.go
index 6a36bae..4a1e133 100644
--- a/src/gx/triangle.go
+++ b/src/gx/triangle.go
@@ -33,14 +33,32 @@ func (t Triangle) SideLengthSquares() ([3]Float) {
 	
 	l1 := LineSegment{t[0], t[1]}.LenSqr()
 	l2 := LineSegment{t[1], t[2]}.LenSqr()
-	l2 := LineSegment{t[2], t[0]}.LenSqr()
+	l3 := LineSegment{t[2], t[0]}.LenSqr()
 	
 	return [3]Float{l1, l2, l3}
 }
 
 // Check whether the point is in the triangle.
 func (t Triangle) PointIsIn(p Point) bool {
-	return false	
+	sls := t.SideLengthSquares()
+	
+	sl0 := LineSegment{p, t[0]}.LenSqr()
+	sl1 := LineSegment{p, t[1]}.LenSqr()
+	sl2 := LineSegment{p, t[2]}.LenSqr()
+	
+	if sl0 > sls[0] || sl0 > sls[2] {
+		return false
+	}
+	
+	if sl1 > sls[0] || sl1 > sls[1] {
+		return false
+	}
+	
+	if sl2 > sls[1] || sl2 > sls[2] {
+		return false
+	}
+	
+	return true
 }
 /*
 func (r *DrawableRectangle) Draw(