With usual notations in $\triangle ABC$,if $a^2+b^2-c^2=ab$,then the measure of angle $C$ is

If in a $\triangle ABC$, $r_1=2$, $r_2=3$ and $r_3=6$, then $a$ equals to

In any $\triangle ABC$,$\frac{\cos 2A}{a^2} - \frac{\cos 2B}{b^2} =$

In $\Delta ABC$,if $\angle C = 90^\circ$,$\angle A = 30^\circ$,and $c = 20$,then the values of $a$ and $b$ are:

In a $\triangle ABC$,if $r_1 = 2r_2 = 3r_3$,then $a : b =$

If in $\Delta ABC$,$AB = 4$,$BC = 6$ and $AC = 5$,and $h_1, h_2, h_3$ are the lengths of the altitudes from vertices $A, B, C$ respectively,then the value of $(\frac{1}{h_1} + \frac{1}{h_2} - \frac{1}{h_3})$ is equal to-