一、引例人教版九年级数学上册第86页例2如下:如图1,⊙O的直径AB为10cm,弦AC为6cm,∠ACB的平分线交⊙O于D,求BC、AD、BD的长.利用勾股定理,不难求得BC=8,而易证△ABD是等腰直角三角形,因而AD=BD=2~(1/2)/2AB=5*2~(1/2).能否求出弦CD的长呢?答案是肯定的.方法1:如图2,过点D作DM⊥CA于点M,作DN⊥BC于点N.容易证明△ADM≌△BDN,△CDM≌△CDN,因此CM= As shown in Figure 1, ⊙ O ⊙ diameter AB is 10cm, chord AC is 6cm, 平 ACB bisector to pay ⊙ O in D, seeking BC, AD, BD long. The use of Pythagorean theorem, it is not difficult to find BC = 8, and easy to prove △ ABD is isosceles right triangle, so AD = BD = 2 ~ (1/2) / 2AB = 5 * 2 ~ (1 / 2) Can I find out the length of the chord CD? The answer is in the affirmative. Method 1: As shown in Figure 2, point D for DM ⊥ CA at point M for DN ⊥ BC at point N. Easy to prove △ ADM ≌ △ BDN, ΔCDM≌ΔCDN, so CM =