1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13 + 14 + 15 + 16 + 17+18 = 171 nhé
1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13 + 14 + 15 + 16 + 17+18. = 171
1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13 + 14 + 15 + 16 + 17+18
3 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13 + 14 + 15 + 16 + 17+18
6 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13 + 14 + 15 + 16 + 17+18
10 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13 + 14 + 15 + 16 + 17+18
15 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13 + 14 + 15 + 16 + 17+18
21 + 7 + 8 + 9 + 10 + 11 + 12 + 13 + 14 + 15 + 16 + 17+18
28 + 8 + 9 + 10 + 11 + 12 + 13 + 14 + 15 + 16 + 17+18
36 + 9 + 10 + 11 + 12 + 13 + 14 + 15 + 16 + 17+18
45 + 10 + 11 + 12 + 13 + 14 + 15 + 16 + 17+18
55 + 11 + 12 + 13 + 14 + 15 + 16 + 17+18
66 + 12 + 13 + 14 + 15 + 16 + 17+18
78 + 13 + 14 + 15 + 16 + 17+18
91 + 14 + 15 + 16 + 17+18
105 + 15 + 16 + 17+18
120+ 16 + 17+18
136+ 17 + 18
153 +18
= 171
1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18=171
Tính nhanh: =1+10+(2+18)+(3+17)+(4+16)+(5+15)+(6+14)+(7+13)+(8+12)+(9+11)
=1+10+20+20+20+20+20+20+20+20
=171
Tính thường:
1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10+ 11 + 12 + 13 + 14 + 15 + 16 + 17 + 18 = 171
1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18=171
1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18
= (1+18)+(2+17)+...........+(9+10)
= 19+19+19+19+19+19+19+19+19
= 19x9
= 171
bn nnhoms nó vào
1+(2+18)+(3+17)+(4+16)+(5+15)+(6+14)+(7+13)+(8+12)+(9+11)+10=171