Question

tính nhanh:

11+13+15+...+99

1+2+3+...+1998+1999

Answer 1

11+13+15+...+99

SSH: (99-11):2+1=45

Tổng: (99+11).45:2=2475

1+2+3+...+1998+1999

SSH: (1999-1):1+1=1999

Tổng: (1999+1).1999:2=1999000