$12)\frac{2-x}{8}=\frac{5-y}{3}=\frac{z+1}{2}$
$⇒\frac{4-2x}{8}=\frac{5-y}{3}=\frac{3z+3}{6}=\frac{(2x-4)+(5-y)-(3z-3)}{6+3-8}$
$=(2x-y-3z)-3=-20-3=-23$
$13)\frac{4}{x-2}=\frac{14}{y-7}=\frac{10}{z-5}$
$⇒\frac{8}{2x-4}=\frac{14}{y-7}=\frac{10}{z-5}=\frac{8+14-10}{(2x-4)+(y-7)-(z+5)}$
$=\frac{12}{(2x+y-z)-6}=\frac{12}{30}=\frac{2}{5}$
$14)$ Làm tương tự ta có:
$\frac{20}{x+4}=\frac{35}{y+7}=\frac{45}{z+9}=\frac{5}{3}$