Bài toán: Tính nhanh
a) 10 – 9 + 8 – 7 + 6 – 5 + 4 -3 + 2 – 1
= (10 – 9) + (8 – 7) + (6 – 5) + (4 -3) + (2 – 1)
= 1 + 1 + 1 + 1 + 1
= 1 x 5 = 5
b) 1 + 3 + 5 + 7 + 9 + 10 + 8 + 6 + 4 + 2 + 0
= (1 + 9) + (3 + 7) + (10 +0) + (8 + 2) + (6 + 4) + 5
= 10 + 10 + 10 + 10 + 10 + 5
= 10 x 5 + 5
= 50 + 5 = 55