Cho

Câu hỏi: Cho $B = \frac{1}{3} + \frac{1}{16} + \frac{1}{19} + \frac{1}{21} + \frac{1}{61} + \frac{1}{72} + \frac{1}{83} + \frac{1}{94}$ . So sánh B với $\frac{3}{5}$

Đáp án

- Hướng dẫn giải

$\begin{array}{l} B = B = \frac{1}{3} + (\frac{1}{16} + \frac{1}{19} + \frac{1}{21}) + (\frac{1}{61} + \frac{1}{72} + \frac{1}{83} + \frac{1}{94}) \\ < \frac{1}{3} + (\frac{1}{15} + \frac{1}{15} + \frac{1}{15}) + (\frac{1}{60} + \frac{1}{60} + \frac{1}{60} + \frac{1}{60}) \\ < \frac{1}{3} + \frac{3}{15} + \frac{4}{60} = \frac{5}{15} + \frac{3}{15} + \frac{1}{15} = \frac{3}{5} \\ \Rightarrow B < \frac{3}{5} \end{array}$