Method #1: Since the only excitation is the unit impulse at time t=0 and the system is causal,
we know that the impulse response before time t=0 is zero. That is. H(t) = 0,t < 0. The homogeneous solution for times t > 0 is of the form 〖ke〗^(-at) and this is the form of the impulse response for times t > 0 because in that time range the system is not being excited We now know the form of the impulse response before timer t = 0 and after time t = 0. All that is left is to find out what happens at time t = 0 The differential equation (6.1) t must be satisfied at all time We can determine what happens at time t=0 integrating both sides of equation (6.2) from t = 0- to t =0+ . infinitesimal times just before and just after zero.