a/

3a−ba+b =3(a+b)−4ba+b =3−4ba+b =34 .

⇒4ba+b =94 ⇒9a+9b=16b⇒9a=7b⇒ab =79 

\(\frac{3a-b}{a+b}=\frac{3}{4}\)

\(\Leftrightarrow4\left(3a-b\right)=3\left(a+b\right)\)

\(\Leftrightarrow12a-4b=3a+3b\)

\(\Leftrightarrow12a-3a=3b+4b\)

\(\Leftrightarrow9a=7b\)

\(\Leftrightarrow\frac{a}{b}=\frac{7}{9}\)

a/

\(\frac{3a-b}{a+b}=\frac{3\left(a+b\right)-4b}{a+b}=3-\frac{4b}{a+b}=\frac{3}{4}.\)

\(\Rightarrow\frac{4b}{a+b}=\frac{9}{4}\Rightarrow9a+9b=16b\Rightarrow9a=7b\Rightarrow\frac{a}{b}=\frac{7}{9}\)

b/

\(\frac{a}{b}=\frac{3}{7}\Rightarrow\frac{a}{3}=\frac{b}{7}=\frac{3a}{9}=\frac{4b}{28}=\frac{3a-4b}{9-28}=\frac{3a-4b}{-19}\)

\(\frac{a}{3}=\frac{b}{7}\Rightarrow\frac{2a}{6}=\frac{3b}{21}\Rightarrow\frac{2a+3b}{6+21}=\frac{2a+3b}{27}\)

\(\Rightarrow\frac{3a-4b}{-19}=\frac{2a+3b}{27}\Rightarrow\frac{3a-4b}{2a+3b}=-\frac{19}{27}\)

=>(3a-b).4=3.(a+b)

=>12a-4b=3a+3b

=>12a-3a=4b+3b

=>9a=7b

=>a/b=7/9

Tĩk nhé

\(\dfrac{3a-b}{a+b}=\dfrac{3}{4}\)

\(\Rightarrow3\left(a+b\right)=4\left(3a-b\right)\)

\(\Rightarrow3\left(a+b\right)-4\left(3a-b\right)=0\)

\(\Rightarrow\left(3a+3b\right)-\left(12a-4b\right)=0\)

\(\Rightarrow3a+3b-12a+4b=0\)

\(\Rightarrow-9a+7b=0\)

\(\Rightarrow-9a=-7b\)

\(\Rightarrow\dfrac{a}{b}=\dfrac{-7}{-9}\)

\(\Rightarrow\dfrac{a}{b}=\dfrac{7}{9}\)

Vậy giá trị tỉ số \(\dfrac{a}{b}=\dfrac{7}{9}\)

toi can giup