Ta có: \(\frac{3a^2+b^2}{a^2+b^2}\)=\(\frac{3}{4}\)
\(\Rightarrow\) (3a2+b2).4=(a2+b2).3
\(\Rightarrow\) 12a2+4b2= 3a2+3b2
\(\Rightarrow\) 12a2-3a2 = -4b2+3b2
\(\Rightarrow\) 9a2= -1b2
\(\Rightarrow\)\(\frac{a^2}{b^2}\)=\(\frac{-1}{9}\)
\(\Rightarrow\)\(\frac{a}{b}\)=\(\frac{-1}{3}\)