c) $\dfrac{x}{19}$ $=$ $\dfrac{y}{21}$ và 2x – y = $34$
d) $\dfrac{x^2}{9}$ $=$ $\dfrac{y^2}{16}$ và x^2 + y^2 = $100$
c) $\dfrac{x}{19}$ $=$ $\dfrac{y}{21}$ và 2x – y = $34$
d) $\dfrac{x^2}{9}$ $=$ $\dfrac{y^2}{16}$ và x^2 + y^2 = $100$
c) $\dfrac{x}{19}=\dfrac{y}{21}$
$↔ \dfrac{2x}{38}=\dfrac{-y}{-21}$
$=\dfrac{2x-y}{38-21}$
$=\dfrac{34}{17}$
$=2$
$\dfrac{x}{19}=2 ↔ x=19.2=38$
$\dfrac{y}{21}=2 ↔ y=21.2=42$
d) $\dfrac{x^2}{9}=\dfrac{y^2}{16}$
$=\dfrac{x^2+y^2}{9+16}$
$=\dfrac{100}{25}$
$=4$
$\dfrac{x^2}{9}=4 ↔ x^2=9.4=36 ↔ x=±6$
$\dfrac{y^2}{16}=4 ↔ y^2=16.4=64 ↔ y=±8$