⨷Âì
¨§¾ÔÊÙ¨¹ìÇèÒ x = 10°
¾ÔÊÙ¨¹ì 1
ãËé AD = L
(1) ∠ACB = 100° áÅÐ ∠ADB = 40°
(2) ¡Ó˹´¨Ø´ P à»ç¹ÀÒ¾Êзé͹¢Í§¨Ø´ D ¼èÒ¹ AB ⇒ ∆ABP ≅ ∆ABD ⇒ BP = BD, ∠ABP = ∠ABD = 30° áÅÐ ∠APB = ∠ADB = 40°
∵ BD = BP áÅÐ ∠DBP = 60° ⇒ ∆BDP à»ç¹ ∆´éÒ¹à·èÒ ⇒ BD = DP
¹Í¡¨Ò¡¹Ñé¹ Âѧä´éÇèÒ ∠BDP = ∠BPD = 60° ⇔ ∠ADP = ∠APD = 20°
(3) ¡Ó˹´¨Ø´ Q º¹ AC ·Õè·ÓãËé DQ = L ⇔ DQ = AD ⇔ ∆ADQ à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠D à»ç¹ÁØÁÂÍ´ ⇔ ∠AQD = ∠DAQ ⇔ ∠AQD = 80° ⇔ ∠ADQ = 20° ⇔ ∠BDQ = 20°
(4) ÊѧࡵÇèÒ ∆BDQ ≅ ∆ADP ´éǤÇÒÁÊÑÁ¾Ñ¹¸ìẺ ´-Á-´ (BD = DP, ∠BDQ = ∠ADP, DQ = AD) ⇒ ∠DBQ = ∠APD ⇔ ∠DBQ = 20° ⇔ ∠DBQ = ∠BDQ ⇔ ∆BDQ à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠Q à»ç¹ÁØÁÂÍ´ ⇔ BQ = DQ ⇔ BQ = L
¾Ô¨ÒÃ³Ò ∆BCQ ¨Ðä´éÇèÒ ∠BQC = 40°
(5) ¾Ô¨ÒÃ³Ò ∆ABC ¨ÐàËç¹ÇèÒ
∠A = 30°, ∠C = 100° áÅÐÁըش Q º¹ AC ·Õè·ÓãËé ∠BQC = 40° ⇒ AC = BQ (Click à¾×èÍ´ÙÇÔ¸Õ¾ÔÊÙ¨¹ìã¹â¨·Âì 1-1) ⇔ AC = L
⇔ AC = AD
⇔ ∆ACD à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠A (= 80°) à»ç¹ÁØÁÂÍ´
⇔ ∠ADC (= ∠ACD) = 50°
⇔ ∠BDC = x = 10°
Q.E.D.
¾ÔÊÙ¨¹ì 2
(1) ∠ACB = 100° áÅÐ ∠ADB = 40°
(2) ¡Ó˹´¨Ø´ P à»ç¹¨Ø´µÑ´ÃÐËÇèÒ§ AC áÅÐ BD
∵ ∠BAP = ∠ABP ⇔ ∆ABP à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠P à»ç¹ÁØÁÂÍ´ ⇔ AP = BP
(3) ¡Ó˹´¨Ø´ Q º¹ AD ·Õè·ÓãËé PQ = AP ⇔ ∆APQ à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠P à»ç¹ÁØÁÂÍ´ ⇔ ∠AQP = ∠PAQ ⇔ ∠AQP = 80° ⇒ ∠APQ = 20° áÅÐ ∠DPQ = 40°
∵ ∠PDQ = ∠DPQ ⇔ ∆DPQ à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠Q à»ç¹ÁØÁÂÍ´ ⇔ DQ = PQ ⇔ DQ = AP
(4) ¡Ó˹´¨Ø´ R º¹ PQ ·Õè·ÓãËé AR = AQ ⇔ ∆AQR à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠A à»ç¹ÁØÁÂÍ´ ⇔ ∠ARQ = ∠AQR ⇔ ∠ARQ = 80° ⇔ ∠ARP = 100°
ÊѧࡵÇèÒ ∆BCP ≅ ∆APR ´éǤÇÒÁÊÑÁ¾Ñ¹¸ìẺ Á-Á-´ (∠BCP = ∠ARP, ∠CBP = ∠APR, BP = AP) ⇒ CP = AR ⇔ CP = AQ
(5) ∵ AP = DQ áÅÐ CP = AQ ⇒ AP + CP = DQ + AQ ⇔ AC = AD ⇔ ∆ACD à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠A (= 80°) à»ç¹ÁØÁÂÍ´ ⇔ ∠ADC (= ∠ACD) = 50° ⇔ ∠BDC = x = 10° Q.E.D.