Tìm x, biết: a) x/3 = 2/3 + -1/7 b) 11/8 + 13/6 = 85/x

* Hướng dẫn giải

a) $\frac{x}{3}=\frac{2}{3}+\frac{-1}{7}$$\iff \frac{x}{3}=\frac{2.7+(-1).3}{21}=\frac{11}{21}\iff x=\frac{11}{7}$

b) $\frac{11}{8}+\frac{13}{6}=\frac{85}{x}\iff \frac{11.3+13.4}{24}=\frac{85}{x}$$\iff \frac{85}{24}=\frac{85}{x}\iff x=24$