过D做DE//AB交BC于E
∵DE//AB
∴∠DEC=∠B = 72°
∵∠C = 36°
∴∠EDC = 72°
∴CD = CE
∵AD//BC
∴四边形ABED是平行四边形
∴BE = AD = 5cm
∴EC = BC-EB = 13-5=8cm
∴CD = CE = 8cm