۱۵. مجموعه‌های زیر را به زبان ریاضی بنویسید. $A = \{5, 10, 15, 20, ...\} = $ $B = \{1, 3, 5, 7, ...\} = $ $C = \{5, 6, 7, 8, 9, 10\} = $ $D = \{1/2, 2/3, 3/4, ...\} = $ $E = \{3, 6, 9, 12, 15, 18\} = $ $F = \{1, 1/2, 1/3, 1/4, 1/5\} = $