If a polygon of $n$ sides has $275$ diagonals,then $n$ is equal to

The longest phase of meiosis is ........

Pepsin is inactivated at $pH$

The angle between the lines $x \cos \alpha_1 + y \sin \alpha_1 = p_1$ and $x \cos \alpha_2 + y \sin \alpha_2 = p_2$ is:

The length of a magnet is large compared to its width and breadth. The time period of its oscillation in a vibration magnetometer is $2 ~s$. The magnet is cut along its length into three equal parts and three parts are then placed on each other with their like poles together. The time period of this combination will be

If $a = \hat{i} + \hat{j} + t \hat{k}$ and $b = \hat{i} + 2 \hat{j} + 3 \hat{k}$,then the values of $t$ for which $(a+b)$ and $(a-b)$ are perpendicular are: