No, a sphere is not a polyhedron.A polyhedron is a three-dimensional geometric figure whose sides are polygons.A regular polyhedron is a polyhedron whose faces are all congruent regular polygons.
Because that is how a polyhedron is defined.
There is no polyhedron with only three sides - a polyhedron being a 3-D shape with plane sides. Once you remove the restriction of plane sides you get several options, some of which are: A cylinder or a cylindrical shape with partial spheres at each end (like a medicine capsule). A segment of a torus (a doughnut with a chunk cut out of it). A spherical wedge - like a segment of a round cheese (that is, real cheese, not the processed plastic that some people consider cheese).
a 3d objet with all flat sides
A Pyramid; Four sides and the base.
No. A polyhedron is 3 dimensional. You have a regular polygon, regular because the sides are equal. If you asked about 20 surfaces, then it would be a polyhedron.
Heptahedron.
A polyhedron is a solid shape with many sides. The word "polyhedron" does not specify a number of sides.
A polyhedron has "many sides" (by definiton). The only 3-D shape which does not have "many sides" is a sphere (which technically only have one "side").
They are the faces of the polyhedron.
It is a polyhedron with two congruent sides: nothing more, nothing less.
The sides of a rectangle are not all equal so a rectangle is not a regular polygon. And, as a consequence, any 3-dimensional shape made from rectangles cannot be a regular polyhedron.
It is not a regular polyhedron because it has unequal sides
A polyhedron with 5 faces could be called a pentahedron, but a more common name would be a quadrilateral pyramid. A square pyramid is a special case of such a shape.
How many sides does it have altogether? If it just has the 2 angles and side and 3 corners, there is no polyhedron with that discription.
No polyhedron has five sides as a pentagon and a hexagon has six sides
I believe the correct term would be a "polyhedron".