1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20
= (1+19) + (2+18) + (3+17) + (4+16) + (5+15) + (6+14) + (7+13) + (8+12) + (9+11) + (10+20)
= 20 + 20 + 20 + 20 + 20 + 20 + 20 + 20 + 20 + 30
= 20 x 9 + 30
= 180 + 30
= 210
Cho mình câu trả lời hay nhất nhé