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), isosceles triangles and 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.

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The reason why triangles are used in architecture is mainly because of its stability.

Triangle stability refers to the stability, firmness and compression resistance of the triangle. For example, the pyramids, railway tracks, triangular frames, cranes, triangular suspenders, roofs, triangular steel frames, steel bridges and Eiffel Tower in Egypt are all built in a triangular shape.

When the lengths of three sides of a triangle are determined, the area and shape of the triangle are completely determined, which is called the stability of the triangle.