设 bc = x
(10-x)2 = 10 * x
100 - 20x + x2 = 10x
x2 - 30x + 100 = 0
x = 15 - 11.18 = 3.82
验证
3.82 * 10 = (10-3.82)2 = 38.2
bc = 3.82 cm