the difference is 12
first we should find out how much is the sum of length and width :
[tex]18 \div2 = 9[/tex]
so we have to find 2 numbers with sum of 9
and least possible area.
the numbers will be 1,8 and the area is
[tex]1 \times 8 = 8[/tex]
now let's find 2 numbers with sum of 9 and greatest possible area.
they are 4 and 5. the area is:
[tex]4 \times 5 = 20[/tex]
the final answer will be the difference between these two areas
[tex]20 - 8 = 12[/tex]
good luck
