When a circular plate of metal is heated in an oven, its radius increases at a rate of .02cm per minute. So the radius DRDT is increasing it's positive at two hundredths centimeters per minute. At what rate is the plates area increasing? When the radius is 50 centimeters? So the R is 50. So we know the formula area equals π R-squared. So the area in terms of time, the derivative of the area in terms of time is DADT and the derivative in terms of time of π R-squared is going to be 2π RDRDT. Remember Π is a constant. So now we're going to stick in R is 50 and the radius changing in terms of time is the two hundredths. When we multiply that out, we should get the area changing as 2π centimeter squared per minute.