[tex]a\leq b\leq c\ (a;b;c-side\ lengths\ of\ the\ triangle)\\\\there\ is\ a\ relationship:\\\\a^2+b^2=c^2-right\ triangle\\\\a^2+b^2 < c^2-obtuse\ triangle\\\\a^2+b^2 > c^2-acute\ triangle[/tex]
[tex]a=5;\ b=9;\ c=12\\\\a^2+b^2=5^2+9^2=25+81=106\\\\c^2=12^2=144\\\\a^2+b^2 < c^2\\\\Answer:Obtuse\ triangle[/tex]