Jika panjang busur BC = 36 cm, panjang busut AC adalah...

180-120 = 60
60 ←→ 36
120 ←→ 120/60×36 = 72cm

Cari sudut CB = 180 - 120 = 60 drjt.
Pakai perbandingan
Sudut AC / CB = AC / CB
120/60 = AC / 36
2 = AC /36
AC = 36 x 2 = 72 cm