Which is the lightest gas?

If $\frac{x^2}{(x^2 + a^2)(x^2 + b^2)} = K \left( \frac{a^2}{x^2 + a^2} - \frac{b^2}{x^2 + b^2} \right)$,then $K =$

An elaborate network of filamentous proteinaceous structures present in the cytoplasm which helps in the maintenance of cell shape is called

$A$ microscope consists of an objective of focal length $1.9 \ cm$ and an eyepiece of focal length $5 \ cm$. The two lenses are kept at a distance of $10.5 \ cm$. If the image is to be formed at the least distance of distinct vision $(25 \ cm)$,the distance at which the object is to be placed before the objective is: (in $cm$)

If the circle $x^2+y^2+2x+3y+1=0$ cuts another circle $x^2+y^2+4x+3y+2=0$ at points $A$ and $B$,then the equation of the circle with $AB$ as a diameter is

The $Ca^{2+}$ and $F^{-}$ ions are located in a $CaF_2$ crystal,respectively at face-centered cubic lattice points and in: