No, there can not be negative amplitude because it is just the distance between the max and the midline or the minimum and the midline.
I see where you might be confused because for instance in the trig equation:
y = -3 sin (.5(x-
/2)) -1, there is a negative in front of the amplitude. However, when finding the amplitude, you take the absolute value of a.
I see where you might be confused because for instance in the trig equation:
y = -3 sin (.5(x-