Try to draw 20, 13, 12 from same point (from left to right), they will make two right triangles with sharing sides on same line (base).
If I can draw a picture for you, you will understand immediately
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谢谢。现在有点懂了::))
-周游列国逍遥人生-
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12/07/2012 postreply
07:45:50
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学绘画图乐::))
-flagsix-
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12/07/2012 postreply
13:36:13