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几天前,学生在“圆”单元测试中遇到这样一道题:“已知从圆外一点引圆的两条切线,这一点与圆心的距离为12,圆的直径为12,则这两条切线的夹角为一度.”在阅卷中发现绝大多数学生的答案为60°,但也有几个学生的答案是60°或120°.到底哪个答案对呢?我一时拿不准,与备课组的老师一齐讨论,从教材(人教版九年级上册)第97页的切线长定理看,两切线的夹角应该是一个,为60°,但仔细想一想,只有60°又不确切、不完善.到底哪个答案正确呢?因为没有这方面的例题和标准答案,老师们也一时拿不准,这一问题成了一个小小的悬题挂了起来.
A few days ago, students in the “round ” unit test encountered such a problem: “known from the outside circle rounding two tangents, this point with the center of the distance of 12, the diameter of the circle is 12, The angle between the two tangents is one degree. ”" The majority of students who found the answer on the marking were 60 °, but there are also a few students who answer 60 ° or 120 °. Are not allowed to discuss together with the teachers in the lesson preparation group. According to the tangent theorem on page 97 of the textbook (Grade Nine of the PEP), the angle between the two tangent lines should be one at 60 °, but think carefully only In the end which answer is correct? Because without the examples and standard answers in this area, the teachers are also not sure at the moment, and this issue has become a small problem hung up.