angles related to rectilinear

Join a line through the two lower points to form a big triangle.
Name the angles as shown:

圖片參考：http://imgcld.yimg.com/8/n/HA05426997/o/701202070089113873435460.jpg

Required answer
= a + b + c + d + e
= a + b + c + (180° - f) (angles sum of triangle) 三角形內角和
= a + b + c + (180° - g) (vertically opposite angle) 對頂角
= a + b + c + (h + i) (angles sum of triangle)
= a + (b + h) + (c + i)
= 180° (angles sum of triangle)

180 degree
ext.angle of triangle
-180
angle sum of quad.


圖片參考：http://imgcld.yimg.com/8/n/HA08244947/o/701202070089113873435470.jpg

180度 ext. angle of triangle


圖片參考：http://imgcld.yimg.com/8/n/HA07126993/o/701202070089113873435460.jpg


2012-02-07 21:08:13 補充：
angle sum of triangle