⨷Âì
¨§¾ÔÊÙ¨¹ìÇèÒ x = 40°
¾ÔÊÙ¨¹ì 1
(1) ∠BDC = 100°
(2) ¡Ó˹´¨Ø´ P à»ç¹ÀÒ¾Êзé͹¢Í§¨Ø´ D ¼èÒ¹ BC ⇒ ∆BCP ≅ ∆BCD ⇒ BP = BD, CP = CD, ∠CBP = ∠CBD = 30° áÅÐ ∠BCP = ∠BCD = 50°
∵ BD = BP áÅÐ ∠DBP = 60° ⇒ ∆BDP à»ç¹ ∆´éÒ¹à·èÒ ⇒ DP = BD ⇔ DP = AC
(3) ¡Ó˹´¨Ø´ Q º¹ BD ·Õè·ÓãËé DQ = CD (= CP) ⇔ ∆CDQ à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠D (= 100°) à»ç¹ÁØÁÂÍ´ ⇔ ∠CQD (= ∠DCQ) = 40°
ÊѧࡵÇèÒ ∆CDQ ≅ ∆CDP ´éǤÇÒÁÊÑÁ¾Ñ¹¸ìẺ ´-Á-´ (CD = CD, ∠CDQ = ∠DCP, DQ = CP) ⇒ CQ = DP ⇔ CQ = AC ⇔ ∆ACQ à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠C à»ç¹ÁØÁÂÍ´ ⇔ ∠CAQ = ∠AQC ⇔ x = 40° Q.E.D.
¾ÔÊÙ¨¹ì 2
(1) µèÍ CD ÍÍ¡ä»Âѧ¨Ø´ P ·Õè·ÓãËé ∠CBP = 50° (⇔ ∠DBP = 20°) ⇔ ∠CBP = ∠BCP ⇔ ∆BCP à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠P à»ç¹ÁØÁÂÍ´ ⇔ BP = CP
¹Í¡¨Ò¡¹Ñé¹ Âѧä´éÇèÒ ∠BDP = 80°
¾Ô¨ÒÃ³Ò ∆BDP ¨Ðä´éÇèÒ ∠BPD = 80° ⇔ ∠BPD = ∠BDP ⇔ ∆BDP à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠B à»ç¹ÁØÁÂÍ´ ⇔ BP = BD ⇔ BP = AC
(2) ¡Ó˹´¨Ø´ Q º¹ BC ·Õè·ÓãËé PQ ⊥ BC ⇒ PQ à»ç¹ÊèǹÊÙ§¢Í§ ∆BCP ⇒ BQ (= CQ) = BC/2
¹Í¡¨Ò¡¹Ñé¹ Âѧä´éÇèÒ ∠BPQ (= ∠CPQ) = (∠BPC)/2 = 40°
(3) ¡Ó˹´¨Ø´ R º¹ AB ·Õè·ÓãËé CR ⊥ AB
¨ÐàËç¹ÇèÒ ∆BCR à»ç¹ ∆ÁØÁ©Ò¡ ·ÕèÁÕ ∠R à»ç¹ÁØÁ©Ò¡ áÅÐ ∠B = 30° ⇒ CR = BC/2
ÊѧࡵÇèÒ ∆ACR ≅ ∆BPQ ´éǤÇÒÁÊÑÁ¾Ñ¹¸ìẺ ©-´-´ (∠ARC = ∠BQP = 90°, CR = BQ, AC = BP) ⇒ ∠CAR = ∠BPQ ⇔ x = 40° Q.E.D.