Optimizing Rectangle for Perimeter

As seen in my previous posts, I know how to optimize a rectangle for
area, but how do I do so for perimeter. Here's my try:

A=xy=100
P=2x+2y

y=100/x

P=2x+200/x
P'=2-200/x^2=0
-200/x^2=-2
-2x^2=-200
x^2=100
x=10

10y=100
y=10

x=10 units
y=10 units
Min P=40 units

Is this correct? Thanks!!!!!

.