Tính 60-61+50-51+40-41+30-31+20-21+10-11+70

60 - 61 + 50 - 51 + 40 - 41 + 30 - 31 + 20 - 21 + 10 - 11 + 70

= (60 - 61) + (50 - 51) + (40 - 41) + (30 - 31) + (20 - 21) + (10 - 11) + 70

= (60 - 61 + 1) + (50 - 51 + 1) + (40 - 41 + 1) + ... + (10 - 11 + 1) + (70 - 1 - 1 - ... - 1)

= 0 + 0 + 0 + 0 + 0 + 0 + (70 - 1 x 6)

= 0 + 70 - 6

= 70 - 6 = 64

60 - 61 + 50 - 51 + 40 - 41 + 30 - 31 + 20 - 21 + 10 - 11 + 70

= (60 - 61) + (50 - 51) + (40 - 41) + (30 - 31) + (20 - 21) + (10 - 11) + 70

= (60 + 1 - 61 ) + (50 + 1  - 51) + (40 + 1 - 41) + ... + (10 + 1 - 11) + (70 - 1 - 1 - ... - 1)

= 0 + 0 + 0 + 0 + 0 + 0+ (70 - 1 x 6)

= 0 + 70 - 6

= 70 - 6 = 64