Question

Bài 1: Tính nhanh 

20+21+22+......+29+30

 

Answer 1

20 + 21 + 22  + ... + 29 + 30

= ( 20 + 30 ) + ( 21+ 29 ) + (22 + 28) + (23+ 27) + (24 + 26) + 25

= 50 + 50 + 50 + 50 + 50 + 25

= 50 x 5 + 25

= 250 + 25

= 275

Answer 2

20+21+22+...+29+30= (30+20)x11:2=275