一、三角形的意义。由不在同一直线上的三条线段首尾顺次连接所组成的封闭图形叫作三角形。三角形有三条边,三个顶点,三个内角,具有稳定性。二、三角形的分类。三角形按角的大小可分为锐角三角形、直角三角形和钝角三角形。锐角三角形的三个角都是锐角(如图1所示);直角三角形一个角是直角,两个角是锐角(如图2所示);钝角三角形一个角是钝角,两个角是 First, the meaning of the triangle. Closed by the end of the three lines are not the same line on the end of the connection consisting of closed graphics called triangles. Triangle has three sides, three vertices, three interior angles, with stability. Second, the classification of the triangle. Triangle by the size of the triangle can be divided into acute triangle, right triangle and obtuse triangle. The three corners of an acute triangle are acute angles (as shown in Figure 1); one of the right triangles is a right angle and the two corners are acute angles (as shown in Figure 2); one of the obtuse triangles is an obtuse angle and the two corners are