SAT推理測驗（SAT Reasoning Test）是美國大學錄取中的一個標準化測試。考察學生的能力是否足夠適應大學教育。

   Read the following SAT test question and then click on a button to select your answer.

   In zhe figure above, the circle with center A and the circle with center C are tangent at point D. If the circles each have radius 10, and if line l is tangent to the circle with center A at point B, what is the value of x？

　　(A) 50

　　(B) 60

　　(C) 63

　　(D) 65

解析Explanation：

   The circles each have radius 10, so A B = A D = D C = 10. Since the circles are tangent at point D, segment Line AC contains D and A C = 20. Also, line A B and l are perpendicular because a line tangent to a circle forms a right angle with the radius at the point of tangency. Therefore, triangle A B C is a right triangle with hypotenuse 20 and side line A B of length 10. A right triangle with one side of length one-half that of its hypotenuse is a 30 degree - 60 degree - 90 degree triangle. The 30 degree angle is opposite side line A B, so x = 90 minus 30 = 60.

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