Tolong sama caranya ya?

                             45°
panjang busur = -------- x 2 π r
                            360°

                              45°
                   11  = -------- x 2 x 3,14 x r
                             360°

                    11 = 0,785 x r
       jari - jari     = 11 / 0,785
                         = 14 cm