A triangle is a fundamental geometric shape consisting of three sides and three angles. It is the simplest polygon in geometry, formed by connecting three points that are not on the same line. Every triangle has three vertices, labeled here as A, B, and C, and three sides connecting these vertices.
Triangles can be classified in different ways. By side lengths, we have equilateral triangles with all sides equal, isosceles triangles with two equal sides, and scalene triangles with all different sides. By angles, triangles can be acute with all angles less than 90 degrees, right triangles with one 90-degree angle, or obtuse triangles with one angle greater than 90 degrees.
三角形是几何学中最基本的平面图形之一。它由三条直线段首尾相连而成，形成一个封闭的平面图形。每个三角形都有三个顶点，分别用字母A、B、C表示，还有三条边和三个内角。
三角形可以按照不同的标准进行分类。按边长分类，有等边三角形、等腰三角形和不等边三角形。按角度分类，有锐角三角形、直角三角形和钝角三角形。这些分类帮助我们更好地理解三角形的性质。
三角形有几个重要的性质。首先，任何三角形的内角和都等于180度。其次，三角形不等式告诉我们，任意一边的长度必须小于其他两边长度的和。三角形的面积等于底乘以高再除以二，周长则是三条边长度的总和。