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Ghép (11;9) ; (12;8) ; ....;(19;1) ta có giá trị mỗi cập là 20
Mà có tất cả: 18/2 = 9 cặp như thế ( do tổng trên có 18 số hạng , 2 số hạng ghép thành một cặp)
===> Tổng trên bằng 20 x 9 =180
11+12+13+.....+18+19+1+2+3+4+.....+8+9
= (11+9)+(12+8)+13+7)+....+(18+2)+(19+1)
= [(19-1)+1.(11+9)
= 19.20
=19.10+19.10
= 380
em mới lớp 6 :D
nếu tính đầy đủ thì = 19 + 1 , 18 + 2 , 17 + 3 , 16 + 4 , 15 + 5 , 14 + 6 , 13 + 7 , 12 + 8 , 11 + 9 = 20 , 20 , 20 , 20 , 20 , 20 , 20 , 20 , 20 20 + 20 + 20 + 20 + 20 + 20 + 20 + 20 + 20 = 40 + 40 + 40 + 40 + 20 = 180 nếu tính tắt thì : 20 x 9 = 180
Ta lấy (1+19)+(2+18)+....(9+11) + 10 + 20
vì 1+19 = 20
mà từ 1 - 19 ( bỏ số 10 ra vì nó chẵn ) có tất cả là 9 cặp
=> 20*9=180 + 10+20 = 210
tk minh nha
=9.887666677000000000000
Toi mat 12 phut de tinh do nha nen ban phai k tui nha
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