When is the '$DUB$' sound heard during the cardiac cycle?

In the following common emitter circuit,if $\beta = 100$,$V_{CE} = 7 \, V$,$V_{BE} = 0 \, V$ (negligible),and $R_C = 2 \, k\Omega$,then $I_B$ is.......$mA$.

If $A$ is the areal velocity of a planet of mass $M$,its angular momentum is

If $\alpha = \frac{5}{2! 3} + \frac{5 \cdot 7}{3! 3^2} + \frac{5 \cdot 7 \cdot 9}{4! 3^3} + \ldots$,then $\alpha^2 + 4\alpha$ is equal to

$A$ ball of mass $1.2 \ kg$ moving with a velocity of $12 \ ms^{-1}$ makes a one-dimensional collision with another stationary ball of mass $1.2 \ kg$. If the coefficient of restitution is $\frac{1}{\sqrt{2}}$,then the ratio of the total kinetic energy of the balls after collision to the initial kinetic energy is:

The vector sum of two forces is perpendicular to their vector difference. In that case,the forces: