Tìm x: ( 1 /3 + 1/15 + 1 /35 + 1/ 63 ) : x = 2/ 3

Ta có: \(\frac{1}{3} + \frac{1}{{15}} + \frac{1}{{35}} + \frac{1}{{63}} = \frac{1}{{1 \times 3}} + \frac{1}{{3 \times 5}} + \frac{1}{{5 \times 7}} + \frac{1}{{7 \times 9}} = \frac{1}{2} \times (1 - \frac{1}{9}) = \frac{1}{2} \times \frac{8}{9} = \frac{4}{9}\)

Do đó có:

\(\frac{4}{9}:x = 2\frac{2}{3} \to x = \frac{4}{9}:1\frac{2}{3} \to x = \frac{4}{9}:\frac{8}{3} \to x = \frac{4}{9} \times \frac{3}{8} \to x = \frac{1}{6}\)

Đáp Số: \(x = \frac{1}{6}\).