Частина 1 — Вправи: 19

19. Поясни обчислення різними способами:

1) 540 + 380 = (500 + 40) + (300 + 80) = (500 + 300) + (40 + 80) = 800 + 120 = 920

2) 54 дес. + 38 дес. = 50 дес. + 30 дес. + 4 дес. + 8 дес. = 80 дес. + 12 дес. = 92 дес. = 920

250 + 670 = (200 + 600) + (50 + 70) = 800 + 120 = 920

180 + 650 = (100 + 600) + (80 + 50) = 700 + 130 = 830

720 + 280 – 570 = (700 + 200 – 500) + (20 + 80 – 70) = (900 – 500) + (100 – 70) = 400 + 30 = 430

360 + 570 = (300 + 500) + (60 + 70) = 800 + 130 = 930

290 + 620 = (200 + 600) + (90 + 20) = 800 + 110 = 910

350 + 270 – 190 = (300 + 200 – 100) + (50 + 70 – 90) = (500 – 100) + (120 – 90) = 400 + 30 = 430

1) 920 – 560 = 920 – (500 + 60) = 920 – 500 – 60 = 420 – 60 = 360

2) 92 дес. – 56 дес. = 80 дес. – 50 дес. + 12 дес. – 6 дес. = 30 дес. + 6 дес. = 36 дес. = 360

610 – 570 = (500 + 110) – (500 + 70) = (500 – 500) + (110 – 70) = 0 + 40 = 40

530 – 360 = 530 – (330 + 30) = 530 – 330 – 30 = 200 – 30 = 170

810 – 350 = 810 – (310 + 40) = 810 – 310 – 40 = 500 – 40 = 460

730 – 540 = 730 – (530 + 10) = 730 – 530 – 10 = 200 – 10 = 190

1000 – (290 + 350) = 1000 – 300 – 340 = 700 – 340 = 360

900 – (380 + 160) = 900 – 400 – 140 = 500 – 140 = 360