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¡Ó˹´ãËé ∠ACD à»ç¹ÁØÁ»éÒ¹
¨§¾ÔÊÙ¨¹ìÇèÒ x = 6°
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(1) ¾Ô¨ÒÃ³Ò ∆BCD ¨Ðä´éÇèÒ ∠ADC = 3x
(2) ¡Ó˹´¨Ø´ P º¹ BC ·Õè·ÓãËé DP = BD ⇔ ∆BDP à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠D à»ç¹ÁØÁÂÍ´ ⇔ ∠BPD = ∠DBP ⇔ ∠BPD = 2x ⇔ ∠CDP = x ⇔ ∠CDP = ∠DCP ⇔ ∆CDP à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠P à»ç¹ÁØÁÂÍ´ ⇔ CP = DP ⇒ CP = BD
(3) ¡Ó˹´¨Ø´ Q à˹×Í AB ·Õè·ÓãËé ∠DAQ = x áÅÐ ∠ADQ = 2x ⇔ ∠CAQ = 30° - x áÅÐ ∠CDQ = x
¨ÐàËç¹ÇèÒ ∆ADQ ≅ ∆BCD ´éǤÇÒÁÊÑÁ¾Ñ¹¸ìẺ Á-´-Á (∠DAQ = ∠BCD, AD = BC, ∠ADQ = ∠CBD) ⇒ AQ = CD áÅÐ DQ = BD
¨ÐàËç¹ÇèÒ ∆CDQ ≅ ∆CDP ´éǤÇÒÁÊÑÁ¾Ñ¹¸ìẺ ´-Á-´ (DQ = DP, ∠CDQ = ∠CDP, CD = CD) ⇒ CQ = CP áÅÐ ∠DCQ = ∠DCP ⇔ CQ = BD áÅÐ ∠DCQ = x
¾Ô¨ÒÃ³Ò ☐ADCQ ¨Ðä´éÇèÒ ∠AQC (ÁØÁãËè) = 360° - 5x ⇔ ∠AQC (ÁØÁàÅç¡) = 5x
(4) ¡Ó˹´¨Ø´ O à»ç¹¨Ø´ÈÙ¹Âì¡ÅÒ§¢Í§Ç§¡ÅÁ·ÕèÁÕ ∆ACD Ṻ㹠⇒ ...
• ¨Ø´ O ÍÂÙèãµé AD
• AO = CO = DO
• ∠AOC = 2(∠ADC) ⇔ ∠AOC = 6x
• ∠COD = 2(∠CAD) ⇔ ∠COD = 60°
∵ CO = DO áÅÐ ∠COD = 60° ⇒ ∆CDO à»ç¹ ∆´éÒ¹à·èÒ ⇔ CD = CO = DO ⇔ AQ = AO
¹Í¡¨Ò¡¹Ñé¹ Âѧä´éÇèÒ ∠CDO = 60° ⇔ ∠ADO = 60° - 3x
∵ AO = DO ⇔ ∆ADO à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠O à»ç¹ÁØÁÂÍ´ ⇔ ∠DAO = ∠ADO ⇔ ∠DAO = 60° - 3x ⇔ ∠OAQ = 60° - 2x
(5) ¡Ó˹´¨Ø´ R à»ç¹ÀÒ¾Êзé͹¢Í§¨Ø´ C ¼èÒ¹ AQ ⇒ ∆AQR ≅ ∆ACQ ⇒ ...
• QR = CQ ⇔ QR = BD
• ∠QAR = ∠CAQ ⇔ ∠QAR = 30° - x ⇔ ∠OAR = 30° - x
• ∠AQR = ∠AQC ⇔ ∠AQR = 5x
ÊѧࡵÇèÒ ∆AOR ≅ ∆AQR ´éǤÇÒÁÊÑÁ¾Ñ¹¸ìẺ ´-Á-´ (AO = AQ, ∠OAR = ∠QAR, AR = AR) ⇒ OR = QR ⇔ OR = BD
¹Í¡¨Ò¡¹Ñé¹ Âѧä´éÇèÒ ∠AOR = ∠AQR ⇔ ∠AOR = 5x ⇔ ∠COR = x
(6) ÊѧࡵÇèÒ ∆COR ≅ ∆CDQ ´éǤÇÒÁÊÑÁ¾Ñ¹¸ìẺ ´-Á-´ (OR = DQ, ∠COR = ∠CDQ, CO = CD) ⇒ CR = CQ
∴ CQ = CR = QR ⇔ ∆CQR à»ç¹ ∆´éÒ¹à·èÒ ⇒ ∠CQR = 60° ⇔ 10x = 60° ⇔ x = 6° Q.E.D.
ËÁÒÂà赯 àÃÒÊÒÁÒöáÊ´§ÇèҨش Q ÍÂÙèã¹ ∆ACD ä´é´Ñ§¹Õé
∵ ∠ACD à»ç¹ÁØÁ»éÒ¹ ⇒ 150° - 3x > 90° ⇔ x < 20° ⇔ ∠DAQ < 20°
∵ ∠DAQ < ∠CAD áÅÐ ∠ADQ < ∠ADC
∴ ¨Ø´ Q ÍÂÙèã¹ ∆ACD