diff --git a/README.md b/README.md
index d8d15f3..2f2e08e 100644
--- a/README.md
+++ b/README.md
@@ -2,3 +2,5 @@ Basic3D
 =======
 
 3D Graphics Library (Basic Classes) for learning
+
+在实际应用中, 构建 3D 场景都会基于比较成熟的图形库, 如[OpenSceneGraph](http://www.openscenegraph.org/)和[Orge](http://www.ogre3d.org/), 或是为了追求效率直接使用 OpenGL 或是 Direct3D. 但无论用哪一种方案, 都毋需担心最底层的数学计算问题. 因为这些库已经或多或少的封装了一些必备数学运算操作. 尤其是关于线性代数方面的运算操作. 但在没有夯实这些数学基础的情况下, 总会在构建场景时遇到一些疑惑, 最典型的就是搞不清楚诸如旋转到底是什么, 怎么转的, 往哪边转之类的问题. 去看底层的源码, 又过分庞杂, 很难快速找到自己感兴趣的点. 这种情况下, 只好去翻书查询基本的数学原理. 但多数的书都偏重于数学公式的推演, 
diff --git a/Vec3UTest.cpp b/Vec3UTest.cpp
index 040e4e9..3985840 100644
--- a/Vec3UTest.cpp
+++ b/Vec3UTest.cpp
@@ -2,7 +2,7 @@
 ///@Brief   Check the exercise of 5.13
 ///@Author  PEZY
 ///@Date    Sept. 2014
-///@See     class B3D::Vec3
+///@See     class B3D::Vec3, "3D Math Primer for Graphics and Game Development"
 ///
 
 #include <iostream>
@@ -29,53 +29,53 @@ int main()
     Vec3 vd(4, -7, 0);
     vd = 3*vd;
     cout << "[" << vd.x << "," << vd.y << "," << vd.z << "]" << endl;
-  
+
     Vec3 ve(4, 5, 0);
     ve /= 2;
-    cout << "[" << ve.x << "," << ve.y << "," << ve.z << "]" << endl; 
+    cout << "[" << ve.x << "," << ve.y << "," << ve.z << "]" << endl;
   }
-  cout << "----------" << endl; 
+  cout << "----------" << endl;
   ///> Exercise 5.13.2
   {
     Vec3 va(-12, 5, 0);
     Vec3 vb(8, -3, 0.5);
     va.normalize();
     vb.normalize();
-    cout << "[" << va.x << "," << va.y << "," << va.z << "]" << endl; 
-    cout << "[" << vb.x << "," << vb.y << "," << vb.z << "]" << endl; 
+    cout << "[" << va.x << "," << va.y << "," << va.z << "]" << endl;
+    cout << "[" << vb.x << "," << vb.y << "," << vb.z << "]" << endl;
   }
-  cout << "----------" << endl; 
+  cout << "----------" << endl;
   ///> Exercise 5.13.3
   {
     Vec3 va1(3, 10, 7);
     Vec3 va2(8, -7, 4);
     Vec3 va = va1 - va2;
-    cout << "[" << va.x << "," << va.y << "," << va.z << "]" << endl; 
+    cout << "[" << va.x << "," << va.y << "," << va.z << "]" << endl;
 
     Vec3 vb1(1, 2, 3);
     Vec3 vb2(2, 10, -6);
     Vec3 vb = 3*vb1 - 4*vb2;
-    cout << "[" << vb.x << "," << vb.y << "," << vb.z << "]" << endl; 
+    cout << "[" << vb.x << "," << vb.y << "," << vb.z << "]" << endl;
   }
-  cout << "----------" << endl; 
+  cout << "----------" << endl;
   ///> Exercise 5.13.4
   {
     Vec3 va1(3, 10, 7);
     Vec3 va2(8, -7, 4);
     cout << B3D::distance(va1, va2)  << endl;
   }
-  cout << "----------" << endl; 
+  cout << "----------" << endl;
   ///> Exercise 5.13.5
   {
     Vec3 vb(-2, 0, 4);
     cout <<  3*vb*(Vec3(8, -2, 1.5) + Vec3(0, 9, 7)) << endl;
-  }  
-  cout << "----------" << endl; 
+  }
+  cout << "----------" << endl;
   ///> Exercise 5.13.8
   {
     Vec3 v = B3D::crossProduct(Vec3(3, 10, 7), Vec3(8, -7, 4));
-    cout << "[" << v.x << "," << v.y << "," << v.z << "]" << endl; 
-  } 
-  cout << "----------" << endl; 
+    cout << "[" << v.x << "," << v.y << "," << v.z << "]" << endl;
+  }
+  cout << "----------" << endl;
   return 0;
 }