If $\underset{n→∞}{\lim}n \cos\frac{π}{4n}\sin\frac{π}{4n} = k$ then k is

$\frac{π}{4}$ $\frac{π}{3}$ $π$ none

$\frac{π}{4}$

$\underset{n→∞}{\lim}n \cos\frac{π}{4n}\sin\frac{π}{4n} = k$ $⇒\underset{n→∞}{\lim}\frac{\sin\frac{π}{2n}}{\frac{2}{n}}= k⇒k=\frac{π}{4}$