The skill of calculating the number of triangles is to mark each smallest triangle, that is, the triangle without other lines in the graph, as 1, 2, 3 from left to right respectively, and the total number of triangles is the sum of the marked 1, 2, 3, which is the answer.

Triangle is a closed figure composed of three line segments on the same plane but not on the same straight line, which has applications in mathematics and architecture. Ordinary triangles are divided into ordinary triangles (three sides are unequal) and isosceles triangles (isosceles triangles with unequal waist and bottom and isosceles triangles with equal waist and bottom, that is, equilateral triangles); According to the angle, there are right triangle, acute triangle and obtuse triangle, among which acute triangle and obtuse triangle are collectively called oblique triangle.

Definition of triangle: A figure surrounded by three line segments (the endpoints of every two adjacent line segments are connected or overlapped) is called a triangle. Draw a vertical line from the vertex of a triangle to its opposite side. The line segment between the vertex and the vertical foot is called the height of the triangle, and the opposite side is called the bottom of the triangle. This triangle is only three stories high. An obtuse triangle can only draw one height, and an obtuse triangle has three heights.

Classification of triangles: acute triangle, right triangle and obtuse triangle according to their angles. Divided by side length: triangle with unequal sides and isosceles triangle (equilateral triangle or regular triangle is a special isosceles triangle). Isosceles triangle: two waists are equal and two base angles are equal. Equilateral triangle: Three sides are equal, and each angle is 60 degrees. (The concepts of top angle, bottom angle, waist and bottom)