Question

If $A=\{x,x\in N\text{and}16-x^2\ge 0\}$, then cardinality of set $A$ is

Answer 1

$16-x^2\ge 0\Rightarrow x^2-16\le 0\Rightarrow (x+4)(x-4)\le 0$
$\Rightarrow x\in [-4,4]$
Since, $x\in N$
$\Rightarrow x\in \{1,2,3,4\}$
$\therefore n\left(A\right)=4$