3.5.
a. $\frac{-6}{x}=\frac{-8}{12}$
⇒ $x= \frac{-6.12}{8} $
⇒ x = 9$
b. $\frac{x}{-7} = \frac{14}{49}$
⇒$x = \frac{14.(-7)}{49}$
⇒ $x = -2$.
3.6.
+ $\frac{4}{x} = \frac{52}{91}$ ⇒ $x= 4 : \frac{52}{91} = 7$
+ $\frac{y}{21} = \frac{52}{91}$ ⇒ $y = 21 . \frac{52}{91} = 12$
+ $\frac{z}{49} = \frac{52}{91}$ ⇒ $z= 49 . \frac{52}{91} = 28$
+ Vậy: $(x; y; z) = (7; 12; 28)$.
3.7.
a. $\frac{-21}{28} = \frac{-39}{52}$
+ $\frac{-21}{28} = \frac{-27:7}{28:7} = \frac{-3}{4}$ $(1)$
+ $\frac{-39}{52} = \frac{-39:13}{52:13} = \frac{-3}{4}$ $(2)$
+ Từ $(1)$ và $(2)$ ⇒ $\frac{-21}{28} = \frac{-39}{52}$.
b. $\frac{-1313}{2121} = \frac{-131313}{212121}$
+ $\frac{-1313}{2121} = \frac{-1313:101}{2121:101} = \frac{-13}{21}$ $(1)$
+ $\frac{-131313}{212121} = \frac{-131313:10101}{212121:10101} = \frac{-13}{21}$ $(2)$
+ Từ $(1)$ và $(2)$ ⇒ $\frac{-1313}{2121} = \frac{-131313}{212121}$.
3.8.
a. $\frac{482 - 39}{567 - 28} = \frac{964 - 78}{1134 - 56}$
+ $\frac{482 - 39}{567 - 28} = \frac{443}{539}$ $(1)$
+ $\frac{964 - 78}{1134 - 56} = \frac{886}{1076} = \frac{443}{539}$ $(2)$
+ Từ $(1)$ và $(2)$ ⇒ $\frac{482 - 39}{567 - 28} = \frac{964 - 78}{1134 - 56}$.
b. $\frac{4563 - 213}{711 - 51} = \frac{1521 - 71}{237 - 17}$
+ $\frac{4563 - 213}{711 - 51} = \frac{4350}{660} = \frac{145}{22}$ $(1)$
+ $\frac{1521 - 71}{237 - 17} = \frac{1450}{220} = \frac{145}{22}$ $(2)$
+ Từ $(1)$ và $(2)$ ⇒ $\frac{4563 - 213}{711 - 51} = \frac{1521 - 71}{237 - 17}$.
3.10.
+ $\frac{-9}{39} = \frac{-3}{13}$.
+ $\frac{12}{9} = \frac{4}{3}$ .
+ $\frac{-35}{10} = \frac{-7}{2}$.
Luyện tập
3.1.
a. $\frac{10}{7} = \frac{100}{70}$
b. $\frac{-7}{8}$ $\frac{-49}{56}$
c. $\frac{4}{3} = \frac{200}{150}$
d. $\frac{-8}{-9} = \frac{72}{81}$
3.2.
a. $\frac{-5}{-6} = \frac{15}{18}$
b. $\frac{33}{y} = \frac{x}{11}$
c. $\frac{390}{1000} = \frac{x}{y}$
d. $\frac{x}{-84} = \frac{17}{y}$
3.3.
a. $6$ phút $= 6 : 60 = \frac{1}{10}$ giờ.
b. $24$ phút $= 24 : 60 = \frac{2}{5}$ giờ.
c. $18$ phút $= 18 : 60 = \frac{3}{10}$ giờ.
d. $50$ phút $= 50 : 60 = \frac{5}{6}$ giờ.
3.4.
a. $\frac{-6}{14}$; $\frac{3}{-7}$; $\frac{-9}{21}$; $\frac{-12}{28}$; $\frac{-15}{35}$.
b. $\frac{-1}{3}$; $\frac{-2}{6}$; $\frac{-3}{9}$; $\frac{-4}{12}$; $\frac{-5}{15}$.
