a, Ta có: $\frac{x}y=\frac{3}4⇔\frac{x}3=\frac{y}4$
Áp dụng t/c dãy tỉ số bằng nhau ta có:
$\frac{x}3=\frac{y}4=\frac{2x}6=\frac{5y}{20}=\frac{2x+5y}{6+20}=\frac{10}{26}=\frac{5}{13}$
$⇒\left \{ {{x=\frac{5}{13}.3=\frac{15}{13}} \atop {y=\frac{5}{13}.4=\frac{20}{13}}} \right.$
b, (2x+3y=7 p ko?)
Ta có: $\frac{2x}{3y}=\frac{-1}3⇔\frac{2x}{-1}=\frac{3y}3$
Áp dụng t/c dãy tỉ số bằng nhau ta có:
$\frac{2x}{-1}=\frac{3y}3=\frac{2x+3y}{-1+3}=\frac{7}2$
$⇒\left \{ {{x=\frac{7}2:2=\frac{7}4} \atop {y=\frac{7}2.3:3=\frac{7}2}} \right.$
c, Ta có: $21x=19y$
$⇔21x-19y=0$
$⇔19(x-y)+2x=0$
$⇔19.4+2x=0$
$⇔x=19.4:2=38$
$⇒y=x-4=38-4=34$
