在圖中，求DE

∠ABD=∠ACE

=>BD//CE

=>△ABD相似於△ACE

=>BD:CE=AD:AE

BD=4cm}

CE=6cm}

AD=5cm}

=>AE=7.5cm}

AD=5cm}

=>DE=2.5cm

2007-03-15 13:36:56 補充：
步驟如下:1)先找出兩相同的角:∠ABD=∠ACE2)從而找出平衡線BD//CE3)再找出兩相似三角形△ABD~△ACE4)列出比例式BD:CE=AD:AE5)根據題目的條件,算出DE=2.5cm

Let x be leght
Step1: 4:6=5:X
Step2:
4 5
-- = --
6 X
Step3:
4X = 30
X = 7.5
So, according to this DE = 7.5-5=2.5
The Answer: DE is 2.5cm