电子课本网 第43页

第43页

信息发布者:
D
$30^{\circ}$
$15^{\circ}$
$\sqrt{3}+2\sqrt{2}$
D
$18^{\circ}$
解: 连接$OA,$过点$O$作$OD\perp AB$于点$D,$$OE\perp AC$于点$E,$则$\angle ODA=\angle OEA = 90^{\circ},$$AD=\frac{1}{2}AB,$$AE=\frac{1}{2}AC。$因为$AB = 2\sqrt{2},$$AC = 2\sqrt{3},$所以$AD=\sqrt{2},$$AE=\sqrt{3}。$因为$\odot O$的半径为$2,$所以$OA = 2,$所以$OD=\sqrt{OA^{2}-AD^{2}}=\sqrt{2},$$OE=\sqrt{OA^{2}-AE^{2}} = 1,$所以$OD = AD,$$OE=\frac{1}{2}OA,$所以$\angle OAD=\angle AOD=\frac{1}{2}(180^{\circ}-\angle ODA)=45^{\circ},$$\angle OAE = 30^{\circ}。$分类讨论如下:
① 如图①,当圆心$O$在$\angle BAC$内部时,$\angle BAC=\angle OAD+\angle OAE = 75^{\circ},$所以$\angle BOC=2\angle BAC = 150^{\circ};$
② 如图②,当圆心$O$在$\angle BAC$外部时,$\angle BAC=\angle OAD-\angle OAE = 15^{\circ},$所以$\angle BOC=2\angle BAC = 30^{\circ}。$
综上所述,$\angle BOC$的度数为$150^{\circ}$或$30^{\circ}。$
解: (1) 过点$O$作$OE\perp AC$于点$E,$则$\angle OEA = 90^{\circ},$$AE=\frac{1}{2}AC。$因为$AC = 2,$所以$AE = 1。$设$\odot O$的半径为$r,$则$OA = r。$因为翻折后点$D$与圆心$O$重合,所以$OE=\frac{1}{2}r,$所以$AE=\sqrt{OA^{2}-OE^{2}}=\frac{\sqrt{3}}{2}r,$所以$\frac{\sqrt{3}}{2}r = 1,$解得$r=\frac{2\sqrt{3}}{3}。$故$\odot O$的半径为$\frac{2\sqrt{3}}{3}。$
(2) 连接$BC。$因为$AB$是$\odot O$的直径,所以$\angle AOB = 180^{\circ},$所以$\angle ACB=\frac{1}{2}\angle AOB = 90^{\circ}。$因为$\angle BAC = 25^{\circ},$所以$\angle B = 90^{\circ}-\angle BAC = 65^{\circ}。$由折叠的性质,得$\overset{\frown}{AD}+\overset{\frown}{CD}=\overset{\frown}{AC},$所以$\angle DCA+\angle BAC=\angle B,$所以$\angle DCA=\angle B-\angle BAC = 40^{\circ}。$