The product of two positive numbers is 72. If one of the numbers is x,

1. Show that the sum of one number and twice the other can be written as $\displaystyle S=x+\frac{144}{x}$( I can do this part)

2.Give a neat sketch of $\displaystyle S=x+\frac{144}{x}$ for x>0. This is the part I have trouble- i cant tell what sort of a graph it is. The answers show a parabola as the solution but I don't know why it is a parabola

3. What is the size of two numbers which will make the value of S least. (again, cant do this one)