Tìm \(a\in N\) để:

a) \(\overline{357a}⋮2\)

b) \(\overline{429a}⋮5\)

c) \(\overline{3a51a}⋮9\)

d) \(\overline{4a231}⋮3\)

e) \(\overline{5a37a}⋮10\)

VD: \(\overline{7a54a1}⋮3\)

\(\overline{7a54a1}⋮3\) khi (7+a+5+4+a+1) \(⋮3\)

\(\Rightarrow\left(17+2a\right)⋮3\)

\(\Rightarrow\left(18+2a-1\right)⋮3\)

Vì \(18⋮3\) nên (2a-1)\(⋮3\) mà a\(\in N\), a\(\le9\) nên \(a\in\left\{2;5;8\right\}\)

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