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26 In the figure above, the diameter is 30, what is the area of shadow?
(1) Angle ACB = 60
(2) AB = 15
27 Is st < 0
(1) s^2t^3 < 0
(2) |s + t| < |s| + |t|
28 Is the quadrilateral ABCD a square
(1) AC = CD
(2) AC ⊥ CD
29 If O and D are centers of two circles, and arc AC is part of the circle D. What is the area of the shadow if AC is perpendicular with BD.
(1) AD = pi
(2) BD = 4
30 An aircraft flew from city P to city Q following the wind with 350 miles per hour, and return from city Q to city P with velocity 250 miles per hour. If the wind velocity between these two cities is constant and if the aircraft return from city Q immediately without landing, what is the average speed of the aircraft during this trip of 800 miles?
答案：
26. D
27. B
28. D
29. D
30. 875/3
