$\frac{a}{b}=\frac{3}{4}$

$\Rightarrow \frac{a}{3}=\frac{b}{4}\Rightarrow \frac{{{a}^{2}}}{{{3}^{2}}}=\frac{{{b}^{2}}}{{{4}^{2}}}=\frac{{{a}^{2}}+{{b}^{2}}}{9+16}=\frac{36}{25}$

*$\frac{{{a}^{2}}}{{{3}^{2}}}=\frac{36}{25}\Rightarrow a=\pm \frac{18}{5}$

*$\frac{{{b}^{2}}}{{{4}^{2}}}=\frac{36}{25}\Rightarrow b=\pm \frac{24}{5}$

Do $\frac{a}{b}=\frac{3}{4}$nên ab cùng dấu. Do đó ab>0.

Suy ra ab = $\frac{18}{5}.\frac{24}{5}=\left( -\frac{18}{5} \right).\left( -\frac{24}{5} \right)=17,28$