In $\triangle ABC$,with usual notations,$\frac{b \sin B - c \sin C}{\sin (B - C)} = $

In $\triangle ABC$,if $r=3$ and $R=5$,then $\frac{1}{ab} + \frac{1}{bc} + \frac{1}{ca} = $

The perimeter of a $\triangle ABC$ is $6$ times the arithmetic mean of the values of the sine of its angles. If its side $BC$ is of unit length,then $\angle A=$

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-

With usual notations,in $\triangle ABC$,the lengths of two sides are $10 \text{ cm}$ and $9 \text{ cm}$ respectively. If angles $A, B, C$ are in $A$.$P$.,then the perimeter of $\triangle ABC$ is:

In $\triangle ABC$,with usual notations,if $\frac{b+c}{11}=\frac{c+a}{12}=\frac{a+b}{13}$,then the value of $\cos A+\cos B+\cos C$ is