與monseigneur的解法類似,利用相似三角形帶來的比例性質 18/(x+18) = S_AEF/S_ACD = (AE*AF)/(AC*AD) = AE/(AE+EC) * AF/(AF+FD) = AG/(AG+BC) * AG/(AG+BD) = (BH/2) / (BH/2+BH/3) * (BH/2)/(BH/2 + 2*BH/3) = 9/35 因此 18/x = 9/(35-9) = 9/26 x = 52