maths

If the difference of two numbers is 14,find the minimum value of the
product of these two numbers.
Sol
x(x－14)
=x^2－14x
=x^2－14x+49－49
=(x－7)^2－49
(x－7)^2>=0
(x－7)^2－49>=－49
x(x－14)>=－49
min=－49

If negative numbers are allowed, the minimum product is
-7 x 7 = -49

If only positive numbers are allowed, the minumum product is
0 x 14 = 0

how to calculate?

2009-12-19 17:10:48 補充：
it is wrong..

Including negative number?

2009-12-19 17:16:56 補充：
To auts123:
0 is not a positive number.