$$
\sum_{k=1}^{n} \sin^2(k) = \frac{n}{2} + \frac{1-\csc(1) \sin(2n+1)}{4}
$$
And since $|\sin(x)| \le 1$ we can say that
$$
\frac{n}{2} + C_1
<\sum_{k=1}^{n} \sin^2(k) < \frac{n}{2} + C_2
$$
where $C_{1}, C_2$ are some constants. Can you conclude from here?
We know that
$$ \sum_{k=1}^{n} \sin^2(k) = \frac{n}{2} + \frac{1-\csc(1) \sin(2n+1)}{4} $$ And since $|\sin(x)| \le 1$ we can say that $$ \frac{n}{2} + C_1 <\sum_{k=1}^{n} \sin^2(k) < \frac{n}{2} + C_2 $$ where $C_{1}, C_2$ are some constants. Can you conclude from here?