Cho 4 số nguyên phân biệt a, b, c, d. Chứng minh rằng (a-b).(a-c).(a-d).(b-c).(b-d).(c-d) chia hết cho 12

What does the doctor recommend to eat?

What do happy people do? 

What kind of food will help people to keep fit and stay healthy?