a: \(7^6+7^5-7^4\)
\(=7^4\left(7^2+7-1\right)\)
\(=7^4\cdot55⋮55\)
b: \(81^7-27^9+3^{29}\)
\(=3^{28}-3^{27}+3^{29}\)
\(=3^{27}\left(3^2+3-1\right)\)
\(=3^{26}\cdot33⋮33\)
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