Ta có: \(A=7^3+7^4+7^5+7^6+...+7^{98}\)
\(\Rightarrow A=\left(7^3+7^4\right)+\left(7^5+7^6\right)+...+\left(7^{97}+7^{98}\right)\)
\(\Rightarrow A=7^3\left(1+7\right)+7^5\left(1+7\right)+...+7^{97}\left(1+7\right)\)
\(\Rightarrow A=7^3.8+7^5.8+...+7^{97}.8\)
\(\Rightarrow A=\left(7^3+7^5+...+7^{97}\right).8⋮8\)
\(\Rightarrow A⋮8\)
Vậy \(A⋮8\)
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A = 73 + 74 + 75 + 76 + ... + 797 + 798
A = (73 + 74) + (75 + 76) + ... + (797 + 798)
A = 73.(1 + 7) + 75.(1 + 7) + ... + 797.(1 + 7)
A = 73.8 + 75.8 + ... + 797.8
A = 8.(73 + 75 + ... + 797)
=> A ⋮ 8.
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