nu e foarte greu, ducem diagonalele in trapez, avem AC=20 cm si:
DC || AB=> ΔOCD~ΔOAB <=> [tex] \frac{OA}{OC} [/tex] = [tex] \frac{OD}{OB} [/tex] = [tex] \frac{DC}{AB} [/tex] = [tex] \frac{3}{7} [/tex] <=> [tex] \frac{0A}{OC} [/tex] = [tex] \frac{3}{7} [/tex] => OA= [tex] \frac{3}{7} [/tex] * OC ,dar avem OA+OC=AC <=> [tex] \frac{3}{7} [/tex] * OC + OC=20 => OC= [tex] \frac{7}{10} [/tex] * 20 = 14 cm
OA= 20-14= 6 cm
