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一、选择题: 1．C 2．B 3．B 4．C 5．6．D 7．B 8．D 9．D 10．(0,3~(1/2)) 1．B(?)A,故B有2~2=8个;2．为四分之一圆;3．用几何意义:S_n为n的二次函数,其图像的对称轴X_0∈
First, multiple choice questions: 1. C 2. B 3. B 4. C 5.6. D 7. B 8. D 9. D 10. (0, 3~(1/2)) 1. B(?)A, so B has 2~2=8; 2. A quarter circle; 3. With geometric meaning: S_n is a quadratic function of n, and the symmetry axis of the image is X_0∈