So,
To answer this question, we could use the fact that:
[tex]\cos (x-\frac{\pi}{2})=\sin x[/tex]Now, given:
[tex]y=9\sin (\frac{4\pi}{3}x)[/tex]We could affirm that this expression is equivalent to:
[tex]y=9\cos (\frac{4\pi}{3}x-\frac{\pi}{2})[/tex]A similar situation occurs to this function:
[tex]y=3\sin (\frac{\pi}{3}x)[/tex]An equivalent cosine function could be:
[tex]y=3\cos (\frac{\pi}{3}x-\frac{\pi}{2})[/tex]