= (11+19) + (12+18) + (13+17) + (14+16) + (10+15)
= 30 + 30 + 30 + 30 + 25 = 145
A = 10 + (11 + 19) + (12 + 18) + (13 + 17) + (14 + 16) + 15
= 10 + 30 + 30 + 30 + 30 + 15 = 145
= (11+19) + (12+18) + (13+17) + (14+16) + (10+15)
= 30 + 30 + 30 + 30 + 25 = 145
A = 10 + (11 + 19) + (12 + 18) + (13 + 17) + (14 + 16) + 15
= 10 + 30 + 30 + 30 + 30 + 15 = 145
Tính nhanh:10+11+12+13+14+15+15+16+17+18+19+20
tinh nhanh
1+2+3+4+5+6+7+8+9+11+12+13+14+15+16+17+18+19
11-12+13-14+15-16+17-18+19
1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20=?
1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20+21=?
Tính: 1+2-3+4-5+6-7+8-9+10-11+12-13+14-15+16-17+18-19+20 = ?
20-19+18-17+16-15+14-13+12-11+10-9+8-7+6-5+4-3+2-1=
Tính:
1 + 2 - 3 + 4 - 5 + 6 - 7 + 8 - 9 + 10 - 11 + 12 - 13 + 14 - 15 + 16 - 17 + 18 - 19 + 20
Tính nhanh:
1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13 + 14 + 15 + 16 + 17 + 18 + 19 + 20