從點到三邊距離和是常數,得方程 b - x + 5x/7 + 8x/7 = x + b - 5x/7 + b - 8x/7 .
b - x = 10x/7 + 16x/7 - 3x = 5x/7. 陰影麵積 = 7 * 5/7 = 5.