求夾角的定義及解釋

When two lines meet at a common point (vertex) the angle between them is called the included angle. The two lines define the angle.
當兩條線交叉時, 所形成的角就稱為夾角(include angle)

參考圖片如下
http://www.mathopenref.com/images/angles/included.gif

2007-03-24 21:38:23 補充：
用vector角度黎解, 兩平面夾角係兩平面normalized 既normal vectors, (a) (b)(a) dot (b) = cos(夾角)平面夾角計法(3-D):e.g.平面1向上, vector(a) = 0i 0j k平面2向左上, 與水平成45度, vector (b) = -0.707i 0j 0.707k(a) dot (b) = 0.707cos(夾角) = 0.707夾角= 45度註: 上z, 右x, 後y

2007-03-24 21:39:08 補充：
當夾角=0時, 即沒有夾角, 因為兩條或平面是平行