Centroid is the average position of all points in a shape. Always lies inside the figure and is the center of mass. Divides each median into two segments, one twice as long
Plans and elevations represent 3D objects from three perspectives. Front elevation shows shape from front, side elevation from side. Plan view shows shape from above looking down
Solids are three-dimensional shapes with volume. Everyday objects are solids, like water bottles and food containers. Same 2D shape can become multiple 3D solids
Rotational symmetry means shapes look identical after partial rotations. Only spheres and circles are fully rotationally symmetric at any angle. Rotations are direct isometries preserving orientation. Rotational symmetry groups are subgroups of Euclidean group
Tetrahedron is a polyhedron with four sides. Isosceles tetrahedron has congruent faces. Regular tetrahedron has equilateral triangle faces. Trirectangular tetrahedron has right-angled triangular faces