Question

Bài 2. Chứng minh:

B= \(\frac{1}{2^2}+\frac{1}{4^2}+\frac{1}{6^2}+...+\frac{1}{100^2}< \frac{1}{2}\)

Answer 1

\(B=\frac{1}{2^2}+\frac{1}{4^2}+\frac{1}{6^2}+....+\frac{1}{100^2}=\frac{1}{4}\left(1+\frac{1}{2^2}+\frac{1}{3^2}+....+\frac{1}{50^2}\right)< \frac{1}{4}\left(1+\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+....+\frac{1}{49.50}\right)\\ =\frac{1}{4}\left(1+\frac{1}{1}-\frac{1}{50}\right)=\frac{1}{4}.\left(2-\frac{1}{50}\right)\\ =\frac{1}{4}.\frac{99}{50}=\frac{99}{200}< \frac{1}{2}\left(\text{đ}pcm\right)\)

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