`b_1)`
`2x=3y`
`⇒x/3=y/2`
`⇒x^2/9=y^2/4=(x^2-y^2)/(9-4)=25/5=5`
`⇒x^2=45⇒x=3\sqrt{5}`
`⇒y^2=20⇒y=2\sqrt{5}`
`b_2)`
`3x=2y=5z`
`⇒(3x)/30=(2y)/(30)=(5z)/30`
`⇒x/10=y/15=z/6=2x/20=(y-2x)/(15-20)=5/-5=-1`
`⇒x=-10`
`⇒y=-15`
`⇒z=-6`
`b_3)`
`4x=5y`
`⇒x/5=y/4 `
Đặt `x/5=y/4=k`
`⇒x=5k`
`⇒y=4k`
`⇒xy=5k.4k=20k^2=80`
`⇒k^2=4`
`⇒k=±2`
`⇒`\(\left[ \begin{array}{l}\left \{ {{x=10} \atop {y=8}} \right.\\\left \{ {{x=-10} \atop {y=-8}} \right.\end{array} \right.\)