calculation in linux Shell

$ mao=$((4+5))

$ echo $mao
9

$ echo expr 4 + 5
expr 4 + 5

$ expr 4 + 5
9

$ $((4+5))
-bash: 9: command not found

$ mao2=expr 4 + 5
-bash: 4: command not found

$ mao2=(expr 4 + 5)
$ echo $mao2
expr