Áp dụng tchat tỉ lệ thức ta có
a) $\dfrac{x}{2} = \dfrac{y}{5} = \dfrac{x+y}{2+5} = \dfrac{-21}{7} = -3$
Vậy $x = -3 . 2 = -6$, $y = -3.5 = -15$.
b) $\dfrac{x}{3} = \dfrac{y}{7} = \dfrac{3x}{9} = \dfrac{2y}{14} = \dfrac{3x-2y}{9-14} = \dfrac{20}{-5} = -4$
Vậy $x = -4.3 = -12$, $y = -4.7 = -28$.
c) $\dfrac{x}{2} = \dfrac{y}{5} = \dfrac{3x}{6} = \dfrac{3x-y}{6-5} = 5$
Vậy $x = 5.2 = 10$, $y = 5.5 = 25$.
d) $\dfrac{x}{5} = \dfrac{y}{3} = \dfrac{3x}{15} = \dfrac{2y}{6} = \dfrac{3x-2y}{15-6} = \dfrac{10}{3}$.
Vậy $x = \dfrac{10}{3} . 5 = \dfrac{50}{3}$, $y = \dfrac{10}{3} . 3 = 10$
e) $\dfrac{x}{3} = \dfrac{y}{6} = \dfrac{x+y}{3+6} = \dfrac{27}{9} = 3$
Vậy $x = 3.3 = 9$, $y = 3.6 = 18$.
f) Từ đề bài ta có
$\dfrac{x}{6} = \dfrac{y}{4} = \dfrac{2x}{12} = \dfrac{5y}{20} = \dfrac{2x-5y}{12-20} = \dfrac{40}{8} = 5$
Vậy $x = 5.6 = 30$, $y = 5.4 = 20$.
g) Từ đề bài ta có
$\dfrac{x}{3} = \dfrac{y}{7} = \dfrac{3x}{9} = \dfrac{3x+y}{9+7} = \dfrac{80}{16} = 5$.
Vậy $x = 5.3 = 15$, $y = 5.7 = 35$.
