Question

how many symmetry planes does a rectangular solid have?

Answer 1

By "rectangular solid" we will mean a "rectangular parallelepiped".

So
this is a polyhedron Q with 6 faces each beibng a ractangular, and each pair of
oposite faces are congruent.

Denote its vertexes by
A,B,C,D, A', B',
C', D'


So that there are 3 pairs of equal faces:

ABCD =
A'B'C'D'
ADD'A' = BCC'B'
ABB'A' = DCC'D'


Then the following
three planes are the symmetry planes of Q:

1) let p1 be the plane passing
through the middle point of AA' and orthogonal to AA'.

2) let p2 be the
plane passing through the middle point of AB and orthogonal to AB.

3) let
p3 be the plane passing through the middle point of AD and orthogonal to
AD.


If all three lengths AA', AB, AD are distinct, then p1, p2, and
p3 are all the symmetry planes.



If, say, AA'=AB and these sides
differs from AD, then there are additional 2 symmetry planes

p4 =
AB'C'D
p5 = A'BCD'


Finally, if all sides are the same AA'=AB=AD,
so Q is a cube, then there are also additional 4 planes:

p6 =
ABC'D'
p7 = A'B'CD

p8 = AA'C'C
p9 = BB'D'D