4. Area of a disk of diameter D : [tex]A= \pi*(D/2)^2\approx3.14*(3.5)^2=38.465[/tex]
Assuming that you use the usual convention of rouding 5 to the greater number :
A=38.47
5. Diameter of a circle of radius r : [tex]A= \pi D=47[/tex] hence [tex]D\approx47/3.14\approx14.97[/tex]
