Step-by-step explanation:
Let y be the width and x be the length.
We have equation for area
Area =Length x Width
20 = Length x Width
[tex]20=xy\\\\y=\frac{20}{x}[/tex]
Now we need to find perimeter of the rectangle as a function of x.
Perimeter = 2 ( Length + Width)
Perimeter = 2 ( x + y)
[tex]\texttt{Perimeter = }x+\frac{20}{x}[/tex]
So Perimeter of the rectangle as a function of x is [tex]x+\frac{20}{x}[/tex]