1. It is known that AB is the diameter of a circle, O is the center of a semicircle, C and E are two points on a circle, and the company CD⊥AB, EF⊥AB, EG ⊥ proves that CD = gf.2, PC tangent circle O is in C, AC is the diameter of a circle, and PEF is the secant of a circle. (2) If ∠ BAC = 600, verify: ah = ao.4, let MN be a straight line outside circle O, let O be OA⊥MN in A, two straight lines drawn from A intersect at B, C, D and E, and straight lines EB and CD intersect at P and Q respectively. Verification: AP = aq.5 Let the midpoint A of MN be two chords BC and DE, and let CD and EB meet MN at P and Q respectively. Proof: AP = AQ.6, Ptolemy theorem: Let ABCD be a convex quadrilateral inscribed with a circle, and prove: ABCD+ADBC = ACBD.7 Make a semicircle with the diameter of AB line, with the center of O and C on a semicircle. So ∠ cab =