Among the electrolytes $Na_2SO_4, CaCl_2, Al_2(SO_4)_3$ and $NH_4Cl,$ the most effective coagulating agent for $Sb_2S_3$ sol is

If $u=\sin ^{-1}\left(\frac{x^4+y^4}{x+y}\right)$, then $x \frac{\partial u}{\partial x}+y \frac{\partial u}{\partial y}$ is equal to

$A$ bi-convex lens is formed with two thin plano-convex lenses as shown in the figure. The refractive index of the first lens is $1.5$ and that of the second lens is $1.2$. Both the curved surfaces have the same radius of curvature $R = 14\; cm$. For this bi-convex lens,if the object distance is $40\; cm$,the image distance will be: (in $; cm$)

In a $\triangle ABC$,if $a \cos^2 \frac{C}{2} + c \cos^2 \frac{A}{2} = \frac{3b}{2}$,then the sides of the triangle are in

If the foot of the perpendicular from $(0,0,0)$ to a plane is $(1,2,3)$,then the equation of the plane is

Which of the following statements is not correct?