First One: Looks like a cosine wave inverted (multiplied by -1) and the AMPLITUDE is reduced by 1/2
soooo: -1/2 cos x
2: Frequency is 1/period period is pi frequency then would be 1/pi
3: Looks like a sine wave but the period is pi .....so the frequency is DOUBLED
sooooo: sin 2x
4: Similar to #3 the frequency is doubled (2x) so the period is HALVED to pi