Let $\,\,\,\,a = 3x + 4y,b = 4x + 3y$ and $c = 5x + 5y$ as $\,\,\,\,x,y > 0,c = 5x + 5y$ is the largest side $\therefore$ $C$ is the largest angle. Now $\cos \,C = {{{{\left( {3x + 4y} \right)}^2} + {{\left( {4x + 3y} \right)}^3} - {{\left( {5x + 5y} \right)}^2}} \over {2\left( {3x + 4y} \right)\left( {4x + 3y} \right)}}$ $= {{ - 2xy} \over {2\left( {3x + 4y} \right)\left( {4x + 3y} \right)}} < 0$ $\therefore$ $C$ is obtuse angle $\Rightarrow \Delta ABC$ is obtuse angled