Which of the following sets is non-empty?

Domain of the function $f(x) = \sqrt{2 - 2x - x^2}$ is

The interval for which ${\sin ^{ - 1}}\sqrt x + {\cos ^{ - 1}}\sqrt x = \frac{\pi }{2}$ holds is:

If $X = \{ 4^n - 3n - 1 : n \in N \}$ and $Y = \{ 9(n - 1) : n \in N \}$,then $X \cup Y$ = . . . . .

Let $A = \{ \theta : \sin \theta = \tan \theta \}$ and $B = \{ \theta : \cos \theta = 1 \}$ be two sets. Then

If $A$ and $B$ are any two non-empty sets and $A$ is a proper subset of $B$. If $n(A) = 4$,then the minimum possible value of $n(A \Delta B)$ is (where $\Delta$ denotes the symmetric difference of set $A$ and set $B$).