1.pada gambar di samping,diketahui

panjang busur = (α° / 360°) x 2 π r
panjang busur ab = (100° / 360°) x 2 x 3,14 x r
                      60   = 1,7 x r
                         r   = 35,3 cm

panjang busur cd = (40° / 360°) x 2 x 3,14 x 35,3
                             = 24,6 cm
                             ≈ 25 cm