四、证明题(16分).设A为圆外一点,AP,AQ为圆的切线,第三条切线交 AP,AQ于E,F,且交切点弦PQ于H,第三条切线的切点为G 求证:(EF,GH)=-1 五、证明题(12分).设 ABC DEF是一条二次曲线的内接六点形,且 ABCD P, CDX EF=Q, DE X AF=L, AF X BC=M, BC X DE=N, EF X AB=R 求证:PL,MQ,RN共点 R3 ç ➶èé➴ (16 ➮). ê A Óëìíä ➱ AP, AQ Óë×îÐ ➱ïðñî Ðò AP, AQ â E, F, ó òîäô P Q â H, ïðñ îÐ × îä Ó G. ÖÑõ (EF, GH) = −1. ö ➶èé➴ (12 ➮). ê ABCDEF ÷ í ñ ❮ø ÏÐ ×ùúûäü ➱ó AB×CD = P, CD × EF = Q, DE × AF = L, AF × BC = M, BC × DE = N, EF × AB = R. ÖÑõ P L, MQ, RN ý äÞ