问题
选择题
Our maths teacher often tells us that failure is the mother of _____.[ ]
A. success
B. successful
C. happiness
D. happy
答案
答案：A
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Our maths teacher often tells us that failure is the mother of _____.[ ]
A. success
B. successful
C. happiness
D. happy
答案：A
(1≤x≤3)绕x轴旋转一周所得的旋转曲面，∑上任一点的法向量与x轴正向夹角大于