The product of two numbers is equal to 28. If one of the numbers is represented by x, the value of the other number is `28/x` .
This gives the sum of the numbers as `S = x + 28/x` .
To determine the minimum value of the sum solve S' = 0 for x; also, the value of S'' for the particular solution of x should be positive.
`S' = 1 - 28/x^2`
`1 - 28/x^2 = 0`
=> `x^2 = 28`
=> `x = +-sqrt 28`
`S'' = 56/x^3`
This is positive for `x = sqrt 28`
The minimum value of the `2*sqrt 28`