⨷Âì
¨§¾ÔÊÙ¨¹ìÇèÒ x = 20°
¾ÔÊÙ¨¹ì (â´Â¤Ø³ ΣΤΑΘΗΣ ΚΟΥΤΡΑΣ)
(1) µèÍ AB ÍÍ¡ä»Âѧ¨Ø´ Q â´Â·Õè AQ = AC ⇒ ∠CBQ = 60°
¨ÐàËç¹ÇèÒ ∆APQ ≅ ∆ACP ´éǤÇÒÁÊÑÁ¾Ñ¹¸ìẺ ´-Á-´ (AQ = AC, ∠PAQ = ∠CAP, AP = AP) ⇒ ∠AQP = ∠ACP ⇔ ∠AQP = 10° ⇔ ∠AQP = ∠PAQ ⇔ ∆APQ à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠P à»ç¹ÁØÁÂÍ´ ⇔ AP = PQ
¾Ô¨ÒÃ³Ò ☐ACPQ ¨Ðä´éÇèÒ ∠CPQ (ÁØÁãËè) = 360° - 40° ⇔ ∠CPQ (ÁØÁàÅç¡) = 40°
(2) ¡Ó˹´¨Ø´ R à˹×Í AQ ·Õè·ÓãËé AR = QR = AQ (= AC) ⇔ ∆AQR à»ç¹ ∆´éÒ¹à·èÒ ⇒ ∠QAR = 60° (⇔ ∠CAR = 40°), ∠AQR = 60° áÅÐ ∠ARQ = 60°
∵ AC = AR ⇔ ∆ACR à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠A (= 40°) à»ç¹ÁØÁÂÍ´ ⇔ ∠ARC (= ∠ACR) = 70° ⇔ ∠CRQ = 10°
¨ÐàËç¹ÇèÒ ∆PQR ≅ ∆APR ´éǤÇÒÁÊÑÁ¾Ñ¹¸ìẺ ´-´-´ (PQ = AP, QR = AR, PR = PR) ⇒ ∠PRQ (= ∠ARP) = (∠ARQ)/2 = 30°
(3) ¡Ó˹´¨Ø´ S à»ç¹¨Ø´µÑ´ÃÐËÇèÒ§ BC ¡Ñº QR
∵ ∠QBS = ∠BQS = 60° ⇒ ∆BQS à»ç¹ ∆´éÒ¹à·èÒ ⇒ BQ = BS
∵ ∠PRS = ∠PCS ⇔ ☐CRPS ÊÒÁÒöṺã¹Ç§¡ÅÁä´é ⇔ ∠CPS = ∠CRS ⇔ ∠CPS = 10° ⇔ ∠QPS = 30°
ÊѧࡵÇèÒ BQ = BS áÅÐ ∠QBS = 2(∠QPS) ⇒ ¨Ø´ B à»ç¹¨Ø´ÈÙ¹Âì¡ÅÒ§¢Í§Ç§¡ÅÁ·ÕèÁÕ ∆PQS Ṻ㹠⇒ BP = BQ ⇔ ∆BPQ à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠B à»ç¹ÁØÁÂÍ´ ⇔ ∠BPQ = ∠BQP ⇔ ∠BPQ = 10° ⇔ ∠ABP = x = 20° Q.E.D.