A Rectangle has 4 sides

and opposite sides of a Rectangle are equal.

Let AB =
*
l
*
& BC = b

So, CD = AB =
*
l
*

& AD = BC = b

Thus,

Perimeter = Sum of all sides

= AB + BC + CD + AD

=
*
l
*
+ b +
*
l
*
+ b

= 2
*
l
*
+ 2b

= 2(
*
l
*
+ b)

So we can write,

Perimeter of rectangle = 2(
*
l
*
+ b)

where
*
l
*
is length and b is width of rectangle