Why don't reflections over a line form a group?

[ktusurveyor]

hey,

how can ı show the reflections over the line do not have group structure?

--reflection of the real plane ------

 

[tiny-tim]

Welcome to PF!

Hi ktusurveyor! Welcome to PF!

(please wait for replies, do not send out PMs)

Hint: what are the conditions for a set to be a group?

which of those conditions are not met by reflections?