Giải câu 2 trang 15 toán VNEN 7 tập 1 .
a) $\frac{11}{12}$ - ($\frac{2}{5}$ + x) = $\frac{2}{3}$ $\Rightarrow$ $\frac{2}{5}$ + x = $\frac{11}{12}$ - $\frac{2}{3}$ $\Rightarrow$ $\frac{2}{5}$ + x = $\frac{1}{4}$ $\Rightarrow$ x = $\frac{1}{4}$ - $\frac{2}{5}$ $\Rightarrow$ x = $\frac{-3}{20}$ ;
b) $\frac{3}{4}$ + $\frac{1}{4}$ : x = $\frac{2}{3}$ $\Rightarrow$ $\frac{1}{4}$ : x = $\frac{2}{3}$ - $\frac{3}{4}$ $\Rightarrow$ $\frac{1}{4}$ : x = $\frac{-1}{12}$ $\Rightarrow$ x = $\frac{1}{4}$ : $\frac{-1}{12}$ $\Rightarrow$ x = -3.