2 câu trả lời
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
