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This part of the extended materials mainly examines the knowledge points of triangles:

A closed figure composed of three line segments that are not on the same straight line is a triangle. A figure surrounded by three straight lines on a plane or three arcs on a sphere, and a graphic plane triangle surrounded by three straight lines; The figure surrounded by three arcs is a spherical triangle, not a triangle.

Judgment method 1:

1, acute triangle: all three internal angles of the triangle are less than 90 degrees.

2. Right triangle: One of the three internal angles of the triangle is equal to 90 degrees, which can be recorded as Rt△.

3. obtuse triangle: one of the three internal angles of the triangle is greater than 90 degrees.

Decision method 2:

1, acute triangle: the largest of the three internal angles of a triangle is less than 90 degrees.

2. Right triangle: The largest of the three internal angles of a triangle is equal to 90 degrees.

3. obtuse triangle: the largest of the three internal angles of a triangle is greater than 90 degrees and less than 180 degrees.