`a^2-3ab+2b^2=0`

`⇒a^2-ab-2ab+2b^2=0`

`⇒a(a-b)-2b(a-b)=0`

`⇒(a-b)(a-2b)=0`

`⇒`\(\left[ \begin{array}{l}a-b=0\\a-2b=0\end{array} \right.\)`⇒`\(\left[ \begin{array}{l}a=b\\a=2b\end{array} \right.\)

`+TH1: a=b`

Mà `a>b`

`⇒a ne b` `(loại)`

`+TH2: a=2b`

`⇒a>b` `(tm)`

Thay `a=2b` vào `Q` ta được:

`Q=(a+b)/(3a-b)=(2b+b)/(3.2b-b)=(3b)/(6b-b)=(3b)/(5b)=3/5`

Vậy `Q=3/5`