⨷Âì
¨§¾ÔÊÙ¨¹ìÇèÒ x = 30°
¾ÔÊÙ¨¹ì
ãËé AP = L
(1) µèÍ AB ÍÍ¡ä»Âѧ¨Ø´ Q â´Â·Õè PQ = L ⇒ ∠CBQ = 140°
∵ AP = PQ ⇔ ∆APQ à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠P à»ç¹ÁØÁÂÍ´ ⇔ ∠AQP = ∠PAQ ⇔ ∠AQP = 10°
(2) µèÍ AC ÍÍ¡ä»Âѧ¨Ø´ R â´Â·Õè PR = L ⇒ ∠BCR = 70°
∵ AP = PR ⇔ ∆APR à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠P à»ç¹ÁØÁÂÍ´ ⇔ ∠ARP = ∠PAR ⇔ ∠ARP = 20°
¾Ô¨ÒÃ³Ò ∆CPR ¨Ðä´éÇèÒ ∠CPR = 80° ⇔ ∠CPR = ∠PCR ⇔ ∆CPR à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠R à»ç¹ÁØÁÂÍ´ ⇔ CR = PR ⇔ CR = L
(3) ¾Ô¨ÒÃ³Ò ☐AQPR ¨Ðä´éÇèÒ ∠QPR (ÁØÁãËè) = 360° - 60° ⇔ ∠QPR (ÁØÁàÅç¡) = 60°
∵ PQ = PR áÅÐ ∠QPR = 60° ⇒ ∆PQR à»ç¹ ∆´éÒ¹à·èÒ ⇒ QR = L áÅÐ ∠PQR = ∠PRQ = 60°
∵ CR = QR ⇔ ∆CQR à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠R (= 80°) à»ç¹ÁØÁÂÍ´ ⇔ ∠QCR = ∠CQR = 50° ⇔ ∠BCQ = ∠BQC = 20° ⇔ ∆BCQ à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠B (= 140°) à»ç¹ÁØÁÂÍ´ ⇔ BC = BQ
(4) ÊѧࡵÇèÒ ∆BQR ≅ ∆BCR ´éǤÇÒÁÊÑÁ¾Ñ¹¸ìẺ ´-´-´ (BQ = BC, BR = BR, QR = CR) ⇒ ∠BRQ (= ∠BRC) = (∠CRQ)/2 = 40° ⇔ ∠BRP = 20°
¾Ô¨ÒÃ³Ò ∆BQR ¨Ðä´éÇèÒ ∠QBR = 70° (⇔ ∠ABR = 110°) ⇔ ∠QBR = ∠BQR ⇔ ∆BQR à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠R à»ç¹ÁØÁÂÍ´ ⇔ BR = QR ⇔ BR = L
∵ BR = PR ⇔ ∆BPR à»ç¹ ∆˹éÒ¨ÑèÇ ·ÕèÁÕ ∠R (= 20°) à»ç¹ÁØÁÂÍ´ ⇔ ∠PBR (= ∠BPR) = 80° ⇔ ∠ABP = x = 30° Q.E.D.