The chloride of an element gives a neutral solution in water. To which group of the periodic table does this element belong?

Give the correct order of electrical conductivity of aqueous solution of the following complexes:
$A. [Pt(NH_3)_6]Cl_4$
$B. [Cr(NH_3)_6]Cl_3$
$C. [Co(NH_3)_4Cl_2]Cl$
$D. K_2[PtCl_6]$

If in a $\triangle ABC$,$\frac{1}{a+c} + \frac{1}{b+c} = \frac{3}{a+b+c}$,then $\angle C$ is equal to (in $^{\circ}$)

The self-inductance of a choke coil is $10 \, mH$. When it is connected to a $10 \, V$ $DC$ source,the power loss is $20 \, W$. When it is connected to a $10 \, V$ $AC$ source,the power loss is $10 \, W$. The frequency of the $AC$ source is ....... $Hz$.

$\int \sqrt{4 \cos ^2 x - 5 \sin ^2 x} \cos x \, dx =$