If the lines $a{x}^2+2hxy+b{y}^2=0$ are equally inclined with the coordinate axes then

Pair of Angle bisector is 

$\begin{array}{l}h\left(x^2-y^2\right)=\left(a-b\right)xy\\ \Rightarrow h{x}^2-\left(a-b\right)xy-h^2{y}^2=0\end{array}$

Pair of Equation of co ordinate axis is xy=0

$\therefore h=0,h^2-ab>0\Rightarrow ab<0$