Vẽ  đồ thị của các hàm số sau : a) y=2x                 b) y = x            c) y = 3x            d) y = -2x               e) y = -x g) y = 1/2x               h)  y  = -1/2x     k)  y = 2/3x

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?