In: Advanced Math
Prove that each element in Pentagon D5 has a unique inverse under the binary operation.
D5={AF, BF, CF, DF, EF,0,72,144,216,288}
Solution:
Set of symmetries of an regular pentagon with vertices levelled as
in anticlockwise order. There are
rotations
(
) where
means rotation about
degrees and
reflections
(
)
where
means reflection about the vertices
.


Observe that

,If
we take an reflection say
,then
rotation about zero degree. this means there is no change in the
original figure, so
is
treated as an identity function.
can
be treated as a bijective function, so it is obvious that we can
take binary operation as composition of function.
[This example shows you
how the operation works.]We can prove the statement through Cayley Table.

From the Cayley table one can say that the inverse element of
that is
, similarly
and
. Here from the Cayley table we conclude that each element of
has
unique inverse element. hence proved.