Tính:

a) HS tự làm.

b)

+ a = 13 cm, b = 12 cm

P = (13 + 12) × 2 = 25 × 2 = 50 (cm)

+ a = 35 km, b = 15 km

P = (35 + 15) × 2 = 50 × 2 = 100 (km)

c)

+ a = 60 cm, b = 30 cm

S = 60 × 30 = 1 800 (cm²)

+ a = 42 cm, b = 21 cm

S = 42 × 21 = 882 (cm²)