At room temperature,a $P-$ type semiconductor has

The covalent alkaline earth metal halide $(X = Cl, Br, I)$ is

Select the correct statement regarding the following change:
$NO_2^+ \to NO_2 \to NO_2^-$

$A$ variable plane passes through a fixed point $(1, 2, 3)$. The foot of the perpendicular from the origin to the plane lies on:

If $f:[-2,2] \rightarrow R$ is defined by $f(x) = \begin{cases} \frac{\sqrt{1+cx}-\sqrt{1-cx}}{x} & \text{for } -2 \leq x < 0 \\ \frac{x+3}{x+1} & \text{for } 0 \leq x \leq 2 \end{cases}$ and is continuous on $[-2,2]$,then $c$ is equal to

The coefficient of $x^k$ in the expansion of $\frac{1-2x-x^2}{e^{-x}}$ is