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a,1-2-3-4-5-6 =6+5+4+3+2+1 =(6+1)+(5+2)+(4+3) =7+ 7+7=7x3=21 b,1+2+3+4+5+6+7+8+9 =(9+1)+(8+2)+(7+3)+(6+4)+5 =10+10+10+10+5=10x4+5=40+5=45
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1+2+3+4+5+6+7+8+9+5
=(1+9)+(2+8)+(3+7)+(4+6)+(5+5)
=10+10+10+10+10
=50
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![](https://rs.olm.vn/images/avt/0.png?1311)
10 - 9 + 8 - 7 + 6 - 5 + 4 - 3 + 2 - 1
= (10 - 9) + (8 - 7) + (6 - 5) + (4 - 3) + (2 - 1)
= 1 + 1 + 1 + 1+ 1 = 5
![](https://rs.olm.vn/images/avt/0.png?1311)
1+2+3+4+5+6+7+8+9
=(1+9)+(2+8)+(3+7)+(4+6)+5
=10+10+10+10+5
=45
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
1+2+3+4+5+6+7+8+9
= (1+9) + (2 +8) + (3 +7 ) +( 4 + 6) + 5
= 10 + 10 + 10 + 10 + 5
= 40 + 5
= 45
1+2+3+4+5+6+7+8+9
=(1+9)+(2+8)+(3+7)+(4+6)+5
=10+10+10+10+5
=40+5=45
Ủng hộ mk nkoa!^_^
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`2 + 3 + 4 + 5 + 6 +7+ 8`
`=(2+8)+(3+7)+(4+6)+5`
`=10+10+10+5`
`=20+10+5`
`=30+5`
`=35`
2 + 3 + 4 + 5 + 6 + 7 + 8
= ( 2 + 8 ) + ( 3 + 7 ) + ( 4 + 6 ) + 5
= 10 + 10 + 10 + 5
= 35