Tìm x biết rằng: \(\frac{x}{3} = \frac{1}{3} + \frac{{ - 1}}{7}\)

* Hướng dẫn giải

Ta có:

\(\begin{array}{l} \frac{1}{3} + \frac{{ - 1}}{7} = \frac{7}{{21}} + \frac{{ - 3}}{{21}} = \frac{{7 + ( - 3)}}{{21}} = \frac{4}{{21}}\\ \frac{x}{3} = \frac{{7{\rm{x}}}}{{21}}\\ = > \frac{{7{\rm{x}}}}{{21}} = \frac{4}{{21}} = > 7{\rm{x}} = 4 = > x = \frac{4}{7} \end{array}\)

Chọn đáp án D