To make a triangle by connecting seven matches end to end is actually to ask how many triangles have a circumference of 7. Because the sum of any two sides of a triangle is greater than the third side, one of the sides can be set to 1 and 2 to get 1, 3, 3 and 2, 2 and 3.
Basic definition
A closed figure composed of three line segments that are not on the same straight line is called a triangle. A figure surrounded by three straight lines on a plane or three arcs on a sphere is called a plane triangle; A figure surrounded by three arcs is called a spherical triangle, also known as a triangle.
A closed geometry consisting of three line segments connected end to end is called a triangle. Triangle is the basic figure of geometric figure.