a, Áp dụng dãy tỉ số bằng nhau, ta có:
\(\frac{x}{y}=\frac{5}{7}\Rightarrow\)\(\frac{x}{5}=\frac{y}{7}=\frac{x+y}{5+7}=\frac{4,05}{12}=\frac{27}{80}\)
\(\Rightarrow\)\(x=\frac{5.27}{80}=\frac{27}{16}; y=\frac{7.27}{80}=\frac{189}{80}\)
b, Có \(\frac{x}{3}=\frac{y}{5}\)\(\Rightarrow\)\(\frac{xy}{3.5}=\frac{y^2}{25}\)\(\Rightarrow\)\(\frac{1215}{15}=\frac{y^2}{25}\)
\(\Rightarrow\)\(y^2=2025\Rightarrow y=\pm45\)
y=45 => x 27
y=-45 => x=-27