Lê Quốc Trần Anh Coordinator
26/07/2017 at 08:57-
Hiệp Dương 26/07/2017 at 09:51Đặt A = 1.2.3 + 2.3.4 + 3.4.5 + ..... + 98.99.100
=> 4A = 1.2.3.4 - 1.2.3.4 + 2.3.4.5 - 2.3.4.5 + ..... + 98.99.100.101
=> 4A = 98.99.100.101
=> A = 98.99.100.101/4
=> A = 24497550
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Aim Egst 26/07/2017 at 20:46b)\(A=1\cdot2\cdot3\cdot4+2\cdot3\cdot4\cdot5+....+27\cdot28\cdot29\cdot30\)
\(5A=1\cdot2\cdot3\cdot4\cdot5+2\cdot3\cdot4\cdot5\cdot\left(6-1\right)+....+27\cdot28\cdot29\cdot30\cdot\left(31-26\right)\)
\(5A=1\cdot2\cdot3\cdot4\cdot5+2\cdot3\cdot4\cdot5\cdot6-1\cdot2\cdot3\cdot4\cdot5+....+27\cdot28\cdot29\cdot30\cdot31-26\cdot27\cdot28\cdot29\cdot30\)
\(5A=27\cdot28\cdot29\cdot30\cdot31\Rightarrow A=\dfrac{27\cdot28\cdot29\cdot30\cdot31}{5}=4077864\)
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Aim Egst 26/07/2017 at 20:43
WaTerFall in Dictionary haven't word "Đặt"