The richest sources of vitamin $B_{12}$ are

The basis of the quantum mechanical model of an atom is

If the system of equations $ax + y + z = 0$,$x + by + z = 0$,and $x + y + cz = 0$ $(a, b, c \neq 1)$ has a non-trivial solution,then the value of $\frac{1}{1 - a} + \frac{1}{1 - b} + \frac{1}{1 - c}$ is:

Let $P(-1, 0)$,$Q(0, 0)$,and $R(3, 3\sqrt{3})$ be three points. Then the equation of the bisector of the angle $\angle PQR$ is

$A$ disc of mass $M$ and radius $R$ is rolling with angular speed $\omega$ on a horizontal plane as shown. The magnitude of angular momentum of the disc about the origin $O$ is

The formula for exponential population growth is