The other side of a three-dimensional triangle is indicated by a dotted line. The painting method is to hook the edges with a black pen, then make some details, and finally show the three-dimensional effect with shadows. Three-dimensional triangle is called triangular pyramid, which is a kind of triangular pyramid. Its geometric shape is composed of four triangles. There is one vertex when the bottom surface is fixed and four vertices when the bottom surface is not fixed. A regular triangular pyramid is different from a regular tetrahedron, and there must be a regular triangle on each face of the regular tetrahedron. 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.