1.D是BC的中点,BD = 1 cm,AD = AB + BD = 1 + 1 = 2 cm
2.BD = CD + BC
CE = CD + DE
BE = BC + CD + DE
BD = AD - AB = BE - DE
3.平角=180°,15° = 1/12 平角
周角=360°,3/8周角 = 360 * 3/8 = 135°
1° = 60',25°15′=25.25°
12.(1) AB > CD
(2) AB = CD
(3) AB < CD
14.如BC在A的同一侧,AD = AB/2 = 8,AE = AC/2 = 20,DE = AE - AD = 20 - 8 = 12 cm
如BC在A的两侧,AD = AB/2 = 8,AE = AC/2 = 20,DE = AE + AD = 20 + 8 = 28 cm
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