Calculus 3 - Double Integrals in Polar Coordinates

The change to polar coordinates is
∫∫ f(rcosθ, rsinθ )rdrdθ
But taking a look at example 2 here: http://tutorial.math.lamar.edu/Classes/CalcIII/DIPolarCoords.aspx it is just simply ∫∫ rdrdθ, so my question is why is it rdrdθ instead of f(rcosθ, rsinθ)rdrdθ on example 2?

Question

Calculus 3 - Double Integrals in Polar Coordinates

The change to polar coordinates is 
∫∫ f(rcosθ, rsinθ )rdrdθ
But taking a look at example 2 here: http://tutorial.math.lamar.edu/Classes/CalcIII/DIPolarCoords.aspx it is just simply ∫∫ rdrdθ, so my question is why is it rdrdθ instead of f(rcosθ, rsinθ)rdrdθ on example 2?

anonymous · Answer

Is it because the two polars equations are given? So you use rdrdθ and when it is not given you use f(rcosθ, rsinθ)rdrdθ, is this correct?