Chứng minh \[B = \frac{1}{{{2^2}}} + \frac{1}{{{3^2}}} + \frac{1}{{{4^2}}} + \frac{1}{{{5^2}}} + \frac{1}{{{6^2}}} + \frac{1}{{{7^2}}} + \frac{1}{{{8^2}}} < 1\].
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