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Bạn lấy số cuối dãy trừ số đầu dãychia khoảng cách la 1 va tru 1 la tim ra so phan tu
Roi ban lay so dau day cong so cuoi daycong phan tu ban vua tinh ra chia 2
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tổng này có:99-10+1=90 so hang
(10,10+99,99)x90:2=4954,05
k nhe
![](https://rs.olm.vn/images/avt/0.png?1311)
\(A=10\cdot11+11\cdot12+12\cdot13+...+29\cdot30\\ 3A=10\cdot11\cdot3+11\cdot12\cdot3+12\cdot13\cdot3+...+29\cdot30\cdot3\\ 3A=10\cdot11\cdot\left(12-9\right)+11\cdot12\cdot\left(13-10\right)+12\cdot13\cdot\left(14-11\right)+...+29\cdot30\cdot\left(31-28\right)\\ 3A=10\cdot11\cdot12-9\cdot10\cdot11+11\cdot12\cdot13-10\cdot11\cdot12+12\cdot13\cdot14-11\cdot12\cdot13+...+29\cdot30\cdot31-28\cdot29\cdot30\\ 3A=\left(10\cdot11\cdot12-10\cdot11\cdot12\right)+\left(11\cdot12\cdot13-11\cdot12\cdot13\right)+...+\left(28\cdot29\cdot30-28\cdot29\cdot30\right)+29\cdot30\cdot31-9\cdot10\cdot11\\ 3A=29\cdot30\cdot31-9\cdot10\cdot11\\ A=\dfrac{29\cdot30\cdot31-9\cdot10\cdot11}{3}\\ A=\dfrac{3\cdot\left(29\cdot10\cdot31-3\cdot10\cdot11\right)}{3}\\ A=29\cdot10\cdot31-3\cdot10\cdot11\\ A=10\cdot\left(29\cdot31-3\cdot11\right)\\ A=10\cdot866\\ A=8660\)
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Đặt A = 10.11 + 11.12 + ... + 29.30
ta có : 3A = 10.11.3 + 11.12.3 + ... + 29.30.3
3A = 10.11.(12-9) + 11.12.(13-10) + ... + 29.30.(31-28)
3A = 10.11.12 - 9.10.11 + 11.12.13 - 10.11.12+ ... + 29.30.31 - 28.29.30
3A = (10.11.12 + 11.12.13 + ... + 29.30.31) - (9.10.11 + 10.11.12 + ... + 28.29.30)
3A = 29.30.31 - 9.10.11
3A = 26970 - 990
3A = 25980
A = 25980 : 3
A = 8660
Vậy A = 8660