a)
AB = 8 cm => MB = 4 cm
BC = 10 cm => BN = 5 cm } => MN = 4 + 5 = 9 cm
b)
AB = 20 cm => MB = 10 cm
BC = 40 cm => BN = 20 cm } => MN = 10 + 20 = 30 cm
c)
AB = 6 cm => MB = 3 cm
BC = 4 cm => BN = 2 cm } => MN = 3 + 2 = 5 cm
d)
AB = 12 dm => MB = 6 dm
BC = 9 dm => BN = 4,5 dm } => MN = 6 + 4,5 = 10,5 dm
e)
AB= 6 mm => MB = 3 mm
BC = 80 mm => BN = 40 mm } => MN = 3 + 40 = 43 mm
f)
BC = 6 cm => BN = 3 cm
MB = (AC - BC) : 2 = (14 - 6) : 2 = 4 cm } => MN = 3 + 4 = 7 cm