[tex]The\ surface\ area\ of\ the\ sphere=4 \pi r^2\\\\The\ surface\ area\ of\ the\ sphere=81 \pi (in^2)\\\\4 \pi r^2=81 \pi\\4r^2=81\ /:4\\\\r^2= \frac{\big{81}}{\big{4}} \ \ \ \Rightarrow\ \ \ r= \sqrt{ \frac{\big{81}}{\big{4}}} = \frac{\big{9}}{\big{2}}=4.5\ (in)\\\\the\ radius\ of\ the\ sphere\ is\ 4.5\ in[/tex]
