Which formula is used to determine the contact area of a drum with a rim?

The contact area of a drum with a rim can be determined using the formula for the area of a circle, given by ( A = \pi r^2 ), where ( r ) is the radius of the rim. If the rim has a specific width, the formula may need to account for the annular area by subtracting the area of the inner circle from the outer circle. Therefore, if ( r_1 ) is the inner radius and ( r_2 ) is the outer radius, the contact area can be calculated as ( A = \pi (r_2^2 - r_1^2) ).