轴对称形状
(1) 20
(2)有，设∠MAB = X∠NAC = Y，则∠ ABM = X ∠ AMN = 2x，∠ ACN = Y，∠ ANM = 2Y，∞。
(3)10cm，AM=BM，AN=CN，BC = BM+MN+CN = AM+MN+AN = C△AMN = 10cm
(2)有，设∠MAB = X∠NAC = Y，则∠ ABM = X ∠ AMN = 2x，∠ ACN = Y，∠ ANM = 2Y，∞。
(3)10cm，AM=BM，AN=CN，BC = BM+MN+CN = AM+MN+AN = C△AMN = 10cm