What is the period of the graph of each of the following functions? Give answers in radians as a fraction of pi where appropriate. \text { (a) } y=2 \cos

\left(\frac{x}{3}\right) \text { (b) } y=4 \sin (\pi x) \text { (c) } y=3 \sin (2 x)+4 \text { (d) } y=-5 \cos \left(\frac{\pi x}{2}\right)