Giải câu 3 trang 31 toán VNEN 7 tập 1 .
a) $\frac{x}{7}$ = $\frac{18}{14}$ $\Rightarrow$ x = $\frac{7.18}{14}$ $\Rightarrow$ x = 9;
b) 6 : x = 1$\frac{3}{4}$ : 5 $\Rightarrow$ x = $\frac{6.5}{1\frac{3}{4}}$ $\Rightarrow$ x = $\frac{120}{7}$;
c) 5,7 : 0,35 = (-x) : 0,45 $\Rightarrow$ -x = $\frac{5,7.0,45}{0,35}$ $\Rightarrow$ -x = $\frac{513}{70}$ $\Rightarrow$ x = -$\frac{513}{70}$.