3) A system is called invertible (Section 1.6.2) if we can determine its input signal x uniquely by observing its output signal y, i.e., x y System Inverse system Determine if each of the following systems is invertible. If the system is invertible, give the inverse system (note: a single counter example is sufficient to prove that a system is not invertible). a) y(t) = 2x(t) b) y(t) = x²(1) c) y(t) = ['_x(7) dr - X d) y[n] = x[k] k = -x e) y[n] = nx[n]