Answer:
7k+1−2k+1⋮5⇔((7k+1−2k+1)−(7k−2k))⋮5
((7k+1−2k+1)−(7k−2k))=7⋅7k−2⋅2k−7k+2k=
=6⋅7k−2k=5⋅7k+(7k−2k)⋮5
5⋅7k⋮5
(7k−2k)⋮5
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