What are regular and irregular solids?

Answer 1

A regular solid is also called a platonic solid. It is a solid whose faces are identical regular polygons. There are 5 such solids. There are only 5 of them because a regular solid has 3, 4 or 5 regular polygons meeting at a vertex. If you look at the maximum number of angles you can see why there are exactly 5 platonic solids. The 5 platonic solid are: Tetrahedron where 3 triangles meet at each vertex, the octahedron where 4 meet at each vertex and the last one made of triangles is the icosahedrons which 5 triangles at each vertex, the cube which has 3 squares meeting at each vertex, and lastly the dodecahedron which is made up of regular pentagons with 3 meet at each vertex. In each case, you can see that 5 is the most number of triangles since 6 would be 6 x 60 degrees >360, 4 squares would be 4 x 90=360, and pentagons have interior angles of 108 degrees so you have (3×108°=324°). Anything more than that is greater than or equal to 360 degrees so not possible. Furthermore, a hexagon has an interior angle of 120 degrees so you cannot have 3 meeting at a vertex.

A very famous mathematician named Euler also has a formula for the number of faces and vertices which if F+V-E=2 and anything more than the 5 regular solids would violate Euler's formula which has been proven to be true.

Solids that are not regular are irregular solids.