Giải thích các bước giải:
Nếu $l=2k$
$\to -4\sin(\dfrac{\pi}{2}+l\cdot \pi)=-4\sin(\dfrac{\pi}{2}+2k\cdot \pi)=-4\sin(\dfrac{\pi}{2})=-4$
Nếu $l=2k+1$
$\to -4\sin(\dfrac{\pi}{2}+l\cdot \pi)=-4\sin(\dfrac{\pi}{2}+(2k+1)\cdot \pi)=-4\sin(\dfrac{3\pi}{2}+2k\pi)=-4\sin(\dfrac{3\pi}{2})=4$