



 
Hi Ashley, The expression that the teacher gave is for the entire surface area of the cone. πr^{2} gives the area of the circular base and πra gives the area of the lateral region, the sloped sides. In this expression however the letter a designates the slant height. There is a development of this expression in Stephen's response to a question a while ago. Stephen uses slightly different notation and gets where S is the slant height. Simplification of this expression yields
For your problem you know the lateral area and the slant height so
Solve for r. Penny  


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