=(1+9)+(2+8)+(3+7)+(4+6)+5+10
=10+10+10+10+5+10
=55
h mk nha
1+2+3+4+5+6+7+8+9+10
= ( 1 + 9 ) + ( 2 + 8 ) + ( 3 + 7 ) + ( 4 + 6 ) + 10 + 5
= 10 + 10 + 10 + 10 + 10 + 5
= 10 x 5 + 5
= 50 + 5
= 55
1+2+3+4+5+6+7+8+9+10
= ( 1 + 9 ) + ( 2 + 8 ) + ( 3 + 7 ) + ( 4 + 6 ) + 10 + 5
= 10 + 10 + 10 + 10 + 10 + 5
= 10 x 5 + 5
= 50 + 5
= 55
mk nha ban
ta phân theo từng nhóm : ( 1+ 2 ) + ( 3 + 4 ) + ( 5 + 6 ) + ( 7 + 8 ) + ( 9 + 10 )
=3 + 7 + 11 + 15 +19
= ( 3 + 7 ) + ( 19 + 11 ) +15
= 10 + 30 + 15 =40 + 15
= 55
1+2+3+4+5+6+7+8+9+10
= (1+9)+(2+8)+(3+7)+(6+4)+10+5
=10+10+10+10+10+5
=55
1+2+3+4+5+6+7+8+9+10
=(1+9)+(2+8)+(7+3)+(6+4)+10+5
=10+10+10+10+10+5
=55