Tính: 12 - 12 + 11 + 10 - 9 + 8 - 7 + 5 - 4 + 3 + 2 -1 =
Tính: 12 – 12 + 11 + 10 – 9 + 8 – 7 + 5 – 4 + 3 + 2 –1 =
12 – 12 + 11 + 10 – 9 + 8 – 7 + 5 – 4 + 3 + 2 – 1
= (12 – 12) + (11 + 10) – (9 – 8) – (7 – 5) – (4 – 3) + (2 – 1)
= 21 – 1 – 2 – 1 + 1
= (21 – 1) – (1 + 2 – 1)
= 20 – 2
= 18.