We know that [tex]\cos(\frac{\pi}{2} - x) = \sin(x)[/tex], so [tex]cos(\frac{5\pi}{2} - a) = \sin(a)[/tex].
We know that [tex]\cos(\frac{\pi}{2} - x) = \sin(x)[/tex], so [tex]cos(\frac{5\pi}{2} - a) = \sin(a)[/tex].