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Ответ:

Правило :   если    [tex]\bf \dfrac{a}{b}=\dfrac{c}{d}[/tex]   ,   то    [tex]\bf a\, d=b\, c[/tex]   .

[tex]\bf \displaystyle 1)\ \ \frac{2}{3}=\frac{x}{6}\ \ \Rightarrow \ \ \ 2\cdot 6=3\cdot x\ \ ,\ \ \ x=\frac{2\cdot 6}{3}\ \ ,\ \ x=4\\\\\\2)\ \ \frac{4}{5}=\frac{12}{x}\ \ \Rightarrow \ \ \ 4\cdot x=5\cdot 12\ \ ,\ \ \ x=\frac{5\cdot 12}{4}\ \ ,\ \ x=15\\\\\\3)\ \ \frac{x}{4}=\frac{7}{8}\ \ \Rightarrow \ \ \ 8\cdot x=4\cdot 7\ \ ,\ \ \ x=\frac{4\cdot 7}{8}\ \ ,\ \ x=3,5\\\\\\4)\ \ \frac{9}{x}=\frac{3}{5}\ \ \Rightarrow \ \ \ 9\cdot 5=3\cdot x\ \ ,\ \ \ x=\frac{9\cdot 5}{3}\ \ ,\ \ x=15[/tex]