⨷Âì 1
¡Ó˹´ãËé x < 35°
¨§¾ÔÊÙ¨¹ìÇèÒ x = 30°
¾ÔÊÙ¨¹ì 1
(1) ∠BCD = 50° - x áÅÐ ∠ADC = 70° - x
(2) µèÍ AC ÍÍ¡ä»Âѧ¨Ø´ P â´Â·Õè CP = DP ⇒ ∠BCP = 20° + x
∵ CP = DP ⇔ ∆CDP à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠P à»ç¹ÁØÁÂÍ´ ⇔ ∠CDP = ∠DCP ⇔ ∠CDP = 70° ⇔ ∠CPD = 40°
ÊѧࡵÇèÒ CP = DP áÅÐ ∠CPD = 2(∠CBD) ⇒ ¨Ø´ P à»ç¹ circumcenter ¢Í§ ∆BCD ⇔ BP = CP = DP
ãËé BP = L ⇒ CP = L áÅÐ DP = L
(3) ¡Ó˹´¨Ø´ Q ãµé AD ·Õè·ÓãËé AQ = L áÅÐ ∠DAQ = 20° + x
¨ÐàËç¹ÇèÒ ∆ADQ ≅ ∆BCP ´éǤÇÒÁÊÑÁ¾Ñ¹¸ìẺ ´-Á-´ (AD = BC, ∠DAQ = ∠BCP, AQ = CP) ⇒ DQ = BP (= L) ⇔ DQ = AQ ⇔ ∆ADQ à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠Q à»ç¹ÁØÁÂÍ´ ⇔ ∠ADQ = ∠DAQ ⇔ ∠ADQ = 20° + x ⇔ ∠AQD = 140° - 2x
(4) ∵ DP = DQ ⇔ ∆DPQ à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠D (= 160°) à»ç¹ÁØÁÂÍ´ ⇔ ∠DPQ = 10° áÅÐ ∠DQP = 10° ⇔ ∠APQ = 30° áÅÐ ∠AQP = 130° - 2x (> 60° à¾ÃÒÐ x < 35°)
(5) ¡Ó˹´¨Ø´ R à»ç¹ÀÒ¾Êзé͹¢Í§¨Ø´ Q ¼èÒ¹ AP ⇒ ∆APR ≅ ∆APQ ⇒ PR = PQ, AR = AQ = L áÅÐ ∠APR = ∠APQ = 30°
∵ PQ = PR áÅÐ ∠QPR = 60° ⇒ ∆PQR à»ç¹ ∆´éÒ¹à·èÒ ⇒ PQ = QR
¹Í¡¨Ò¡¹Ñé¹ Âѧä´éÇèÒ ∠PQR = 60° ⇔ ∠AQR = 70° - 2x
(6) ÊѧࡵÇèÒ ∆AQR ≅ ∆DPQ ´éǤÇÒÁÊÑÁ¾Ñ¹¸ìẺ ´-´-´ (AQ = DP, AR = DQ, QR = PQ) ⇒ ∠AQR = ∠DPQ ⇔ 70° - 2x = 10° ⇔ x = 30° Q.E.D.
¾ÔÊÙ¨¹ì 2
(1) ∠BCD = 50° - x
(2) µèÍ AC ÍÍ¡ä»Âѧ¨Ø´ P â´Â·Õè CP = DP ⇒ ∠BCP = 20° + x
∵ CP = DP ⇔ ∆CDP à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠P à»ç¹ÁØÁÂÍ´ ⇔ ∠CDP = ∠DCP ⇔ ∠CDP = 70° ⇔ ∠CPD = 40°
ÊѧࡵÇèÒ CP = DP áÅÐ ∠CPD = 2(∠CBD) ⇒ ¨Ø´ P à»ç¹ circumcenter ¢Í§ ∆BCD ⇔ BP = CP = DP
ãËé BP = L ⇒ CP = L áÅÐ DP = L
(3) ¡Ó˹´¨Ø´ Q ãµé AD ·Õè·ÓãËé DQ = L áÅÐ ∠ADQ = 20° + x
¨ÐàËç¹ÇèÒ ∆ADQ ≅ ∆BCP ´éǤÇÒÁÊÑÁ¾Ñ¹¸ìẺ ´-Á-´ (AD = BC, ∠ADQ = ∠BCP, DQ = CP) ⇒ AQ = BP ⇔ AQ = L
(4) ¡Ó˹´¨Ø´ R º¹ AP ·Õè·ÓãËé DR = L ⇔ DR = DP ⇔ ∆DPR à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠D à»ç¹ÁØÁÂÍ´ ⇔ ∠DRP = ∠DPR ⇔ ∠DRP = 40° ⇔ ∠ADR = 40° - x
(5) ∵ DQ = DR áÅÐ ∠QDR = 60° ⇒ ∆DQR à»ç¹ ∆´éÒ¹à·èÒ ⇒ QR = L áÅÐ ∠DQR = 60°
∵ AQ = DQ = QR ⇔ ¨Ø´ Q à»ç¹ circumcenter ¢Í§ ∆ADR ⇒ ∠DAR = (∠DQR)/2 ⇔ x = 30° Q.E.D.
⨷Âì 2
¡Ó˹´ãËé x > 35°
¨§¾ÔÊÙ¨¹ìÇèÒ x = 40°
¾ÔÊÙ¨¹ì
(1) ∠BCD = 50° - x áÅÐ ∠ADC = 70° - x
(2) µèÍ AC ÍÍ¡ä»Âѧ¨Ø´ P â´Â·Õè CP = DP ⇒ ∠BCP = 20° + x
∵ CP = DP ⇔ ∆CDP à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠P à»ç¹ÁØÁÂÍ´ ⇔ ∠CDP = ∠DCP ⇔ ∠CDP = 70° ⇔ ∠CPD = 40°
ÊѧࡵÇèÒ CP = DP áÅÐ ∠CPD = 2(∠CBD) ⇒ ¨Ø´ P à»ç¹ circumcenter ¢Í§ ∆BCD ⇔ BP = CP = DP
ãËé BP = L ⇒ CP = L áÅÐ DP = L
(3) ¡Ó˹´¨Ø´ Q ãµé AD ·Õè·ÓãËé AQ = L áÅÐ ∠DAQ = 20° + x
¨ÐàËç¹ÇèÒ ∆ADQ ≅ ∆BCP ´éǤÇÒÁÊÑÁ¾Ñ¹¸ìẺ ´-Á-´ (AD = BC, ∠DAQ = ∠BCP, AQ = CP) ⇒ DQ = BP (= L) ⇔ DQ = AQ ⇔ ∆ADQ à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠Q à»ç¹ÁØÁÂÍ´ ⇔ ∠ADQ = ∠DAQ ⇔ ∠ADQ = 20° + x ⇔ ∠AQD = 140° - 2x
(4) ∵ DP = DQ ⇔ ∆DPQ à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠D (= 160°) à»ç¹ÁØÁÂÍ´ ⇔ ∠DPQ = 10° áÅÐ ∠DQP = 10° ⇔ ∠APQ = 30° áÅÐ ∠AQP = 130° - 2x (< 60° à¾ÃÒÐ x > 35°)
(5) ¡Ó˹´¨Ø´ R à»ç¹ÀÒ¾Êзé͹¢Í§¨Ø´ Q ¼èÒ¹ AP ⇒ ∆APR ≅ ∆APQ ⇒ PR = PQ, AR = AQ = L áÅÐ ∠APR = ∠APQ = 30°
∵ PQ = PR áÅÐ ∠QPR = 60° ⇒ ∆PQR à»ç¹ ∆´éÒ¹à·èÒ ⇒ PQ = QR
¹Í¡¨Ò¡¹Ñé¹ Âѧä´éÇèÒ ∠PQR = 60° ⇔ ∠AQR = 2x - 70°
(6) ÊѧࡵÇèÒ ∆AQR ≅ ∆DPQ ´éǤÇÒÁÊÑÁ¾Ñ¹¸ìẺ ´-´-´ (AQ = DP, AR = DQ, QR = PQ) ⇒ ∠AQR = ∠DPQ ⇔ 2x - 70° = 10° ⇔ x = 40° Q.E.D.