a=pir^2
c=2pir
a=pir^2=16
pir^2=16
divide both sides by pi or about 3.141592
r^2=5.0929581789406507446042804279205
sqrt both sides
r=2.2567583341910251477923178062431
circumference=2pir
circumference=2pi(2.2567583341910251477923178062431)
c=4.5135166683820502955846356124862pi
about 4.5pi square cm