⨷Âì
¨§¾ÔÊÙ¨¹ìÇèÒ x = 10°
¾ÔÊÙ¨¹ì (â´Â¤Ø³ Javi Master Jans)
(1) ∠ACB = 60° áÅÐ ∠ADB = 40°
∵ ∠BAD = ∠ABD ⇔ ∆ABD à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠D à»ç¹ÁØÁÂÍ´ ⇔ AD = BD
(2) ¡Ó˹´¨Ø´ P º¹ AB ·Õè·ÓãËé CP = BC ⇔ ∆BCP à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠C à»ç¹ÁØÁÂÍ´ ⇔ ∠BPC = ∠CBP ⇔ ∠BPC = 80° ⇒ ∠ACP = 40° áÅÐ ∠APC = 100°
∵ ∠CAP = ∠ACP ⇔ ∆ACP à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠P à»ç¹ÁØÁÂÍ´ ⇔ AP = CP ⇔ AP = BC
(3) ¡Ó˹´¨Ø´ Q à˹×Í AP ·Õè·ÓãËé AQ = PQ = AP (= BC) ⇒ ∆APQ à»ç¹ ∆´éÒ¹à·èÒ ⇒ ∠PAQ = 60° áÅÐ ∠APQ = 60° ⇔ ∠DAQ = 10° áÅÐ ∠CPQ = 40°
∵ CP = PQ ⇔ ∆CPQ à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠P (= 40°) à»ç¹ÁØÁÂÍ´ ⇔ ∠PCQ = ∠CQP = 70°
(4) ÊѧࡵÇèÒ ∆ADQ ≅ ∆BCD ´éǤÇÒÁÊÑÁ¾Ñ¹¸ìẺ ´-Á-´ (AD = BD, ∠DAQ = ∠CBD, AQ = BC) ⇒ ∠ADQ = ∠BDC ⇔ ∠ADQ = x ⇔ ∠BDQ = 40° - x
¹Í¡¨Ò¡¹Ñé¹ Âѧä´éÇèÒ DQ = CD ⇔ ∆CDQ à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠D (= 40°) à»ç¹ÁØÁÂÍ´ ⇔ ∠DCQ = ∠CQD = 70°
(5) ÊѧࡵÇèÒ ∆CDQ ≅ ∆CPQ ´éǤÇÒÁÊÑÁ¾Ñ¹¸ìẺ Á-´-Á (∠DCQ = ∠PCQ, CQ = CQ, ∠CQD = ∠CQP) ⇒ CD = CP ⇔ CD = BC ⇔ ∆BCD à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠C à»ç¹ÁØÁÂÍ´ ⇔ ∠BDC = ∠CBD ⇔ x = 10° Q.E.D.