A = 2 + 2² + 2³ + ... + 2⁵⁶
= (2 + 2² + 2³ + 2⁴) + (2⁵ + 2⁶ + 2⁷ + 2⁸) + ... + (2⁵³ + 2⁵⁴ + 2⁵⁵ + 2⁵⁶)
= 30 + 2⁴.(2 + 2² + 2³ + 2⁴) + ... + 2⁵².(2 + 2² + 2³ + 2⁴)
= 30 + 2⁴.30 + ... + 2⁵².30
= 30.(1 + 2⁴ + ... + 2⁵²)
= 5.6.(1 + 2⁴ + ... + 2⁵²) ⋮ 5
Vậy A ⋮ 5
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