а) 12 × (70 + b) = 12 × 70 + 12 × b = 840 + 12b
б) 38 × (21 – 9) = 38 × 21 – 38 × 9 = 798 – 342 = 456
в) (25 + 250) × 4 = 25 × 4 + 250 × 4 = 100 + 1000 = 1100
г) 6 × (13 + 150) = 6 × 13 + 6 × 150 = 78 + 900 = 978
д) 8 × 29 + 8 × 21 = 8 × (29 + 21) = 8 × 50 = 400
е) 63 × 237 + 63 × 763 = 63 × (237 + 763) = 63 × 1000 = 63000
Ответ: a) 840 + 12b; б) 456; в) 1100; г) 978; д) 400; е) 63000.