What is the diameter of a circle whose area is 16pi?

##8##
Use the formula for the area of a circle:
##A=pir^2##
Here the area is ##16pi##:
##16pi=pir^2##
Divide both sides by ##pi##:
##16=r^2##
Take the square root of both sides:
##sqrt16=sqrt(r^2)##
##4=r##
Since the radius of the circle is ##4## the diameter is twice that:
##d=4xx2=8##