Tính nhanh: 11 + (- 13) + 15 + (- 17) + ...... + 59 + (- 61)
Tính nhanh: 11 + (– 13) + 15 + (– 17) + ...... + 59 + (– 61).
11 + (– 13) + 15 + (– 17) + ...... + 59 + (–61)
= (–2) + (–2) + … + (–2)
= (–2) . 13
= –26.