4. Найди неизвестный член пропорции: a) 3,2 : 2,8 = y : 2,1; z 105 b) =70 6 1 c) y: = 1 : 3 ;

a) 3,2 : 2,8 = y : 2,1 $$y = \frac{3.2 \cdot 2.1}{2.8} = \frac{3.2 \cdot 21}{28} = \frac{3.2 \cdot 3}{4} = 0.8 \cdot 3 = 2.4$$ b) $$\frac{z}{4} = \frac{105}{70}$$ $$z = \frac{4 \cdot 105}{70} = \frac{4 \cdot 15}{10} = \frac{2 \cdot 15}{5} = 2 \cdot 3 = 6$$ c) $$y : \frac{6}{7} = 1\frac{1}{6} : 3\frac{1}{2}$$ $$y : \frac{6}{7} = \frac{7}{6} : \frac{7}{2}$$ $$y = \frac{6}{7} \cdot \frac{7}{6} : \frac{7}{2} = \frac{6}{7} \cdot \frac{7}{6} \cdot \frac{2}{7} = 1 \cdot \frac{2}{7} = \frac{2}{7}$$ Ответ: a) y = 2.4; b) z = 6; c) y = 2/7.