g, $\frac{x+1}{3}$ =$\frac{y+2}{4}$ =$\frac{z+3}{5}$ và x+y+z=18
=>$\frac{x+y+z+(1+2+3)}{3+4+5}$ =$\frac{18+6}{12}$ =2
$\frac{x+1}{3}$ =2
=>x+1=2.3
=>x+1=6
=>x=6-1=5
$\frac{y+2}{4}$ =2
=>y+2=2.4
=>y+2=8
=>y=8-2=6
$\frac{z+3}{5}$ =2
=>z+3=2.5
=>z+3=10
=>z=10-3=7
i,đặt $\frac{x}{2}$ =$\frac{y}{3}$= $\frac{z}{5}$ =k
=>x=2k; y=3k;z=5k
thay x=2k;y=3k;z=5k vào xyz=-30 ta được ;
2k.3k.5k=-30
=>30$k^{3}$= -30
=>$k^{3}$ =-30:30
=>$k^{3}$=-1
=> $k^{3}$ =$(-1)^{3}$
=>k=-1
=>x=-1.2=-2
y=-1.3=-3
z=-1.5=-5
