In the closest packing of atoms,there are

An $X$-ray tube is operated at $50 \ kV$. The minimum wavelength produced is ..... $\mathring{A}$.

$A$ particle moves along the curve $y=x^2+2x$. The point on the curve such that the $x$ and $y$ coordinates of the particle change at the same rate is

When two related populations occupy geographically or spatially separate areas,they are called

The equation of the parabola with the focus $(3,0)$ and the directrix $x+3=0$ is:

$A$ vertical pole subtends an angle $\tan ^{-1}\left(\frac{1}{2}\right)$ at a point $P$ on the ground. If the angles subtended by the upper half and the lower half of the pole at $P$ are respectively $\alpha$ and $\beta$,then $(\tan \alpha, \tan \beta)$ is equal to