Thực hiện phép tính (tính nhanh nếu có thể): a) 3/5 +- 4/9; 2. Tìm x: a) x + 1/3 = 5/12;
Giải thích
1.
a) \[\frac{3}{5} + \frac{{ - 4}}{9}\]\[ = \frac{{27}}{{45}} + \frac{{ - 20}}{{45}}\]\[ = \frac{7}{{45}}\];
b) \[\frac{{ - 3}}{7} \cdot \frac{2}{5} + \frac{{ - 3}}{7} \cdot \frac{3}{5}\]\[ = \frac{{ - 3}}{7} \cdot \left( {\frac{2}{5} + \frac{3}{5}} \right)\]\[ = \frac{{ - 3}}{7} \cdot 1\]\[ = \frac{{ - 3}}{7}\].
2.
a) \[x + \frac{1}{3} = \frac{5}{{12}}\] \[x = \frac{5}{{12}} - \frac{1}{3}\] \[x = \frac{5}{{12}} - \frac{4}{{12}}\] \[x = \frac{1}{{12}}\] Vậy \[x = \frac{1}{{12}}\]. | b) \(\frac{2}{3}x + \frac{1}{2}x = \frac{{15}}{{12}}\) \(\left( {\frac{2}{3} + \frac{1}{2}} \right)\,\,.\,\,x = \frac{{15}}{{12}}\) \(\frac{7}{6}x = \frac{{15}}{{12}}\) \(x = \frac{{15}}{{12}}:\frac{7}{6}\) \(x = \frac{{15}}{{14}}\). Vậy \(x = \frac{{15}}{{14}}\). |