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北师大版教科书《数学》九年级上册第一章讲了三个重要的定理：1．等腰三角形的两个底角相等；2．有两个角相等的三角形是等腰三角形；3．在直角三角形中,如果一个锐角等于30°,那么它所对的直角边等于斜边的一半．这三个定理讲的不仅仅是三角形的性质和判定,也是一种重要的转化思想,是将角与角的关系转化为线段与线段间的关系,或将线段与线
Beijing Normal University’s textbook “Mathematics” grade nine on the first chapter discusses three important theorems: 1. The two base angles of isosceles triangles are equal; 2. Two triangles with equal angles are isosceles triangles; 3. In a right-angled triangle, if an acute angle is equal to 30°, then the right-angled edge is equal to half of the hypotenuse. These three theorems are not only about the nature and judgment of the triangle, but also an important transformation idea. They are the transformation of the relationship between the angle and the angle into the relationship between the line segment and the line segment, or the line segment and the line.