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三角形中边角不等关系的证明是一类常见的几何证明题型．在这类题的证明中往往用到以下定理或性质；（Ⅰ）垂线段最短；（Ⅱ）三角形中任意两边之和大于第三边；（Ⅲ）三角形的一个外角大于与它不相邻的任意一个内角；（Ⅳ）在同一个三角形中，大边对大角；（Ⅴ）在同一个三角
The proof of the unequal relations between corners in a triangle is a common type of geometric proof. The following theorems or properties are often used in the proof of such questions: (I) The vertical line is the shortest; (II) The sum of any two sides of a triangle is greater than the third edge; (III) An external angle of a triangle is larger than it is not adjacent to it. Any interior angle; (IV) in the same triangle, large to large; (V) in the same triangle