question
3. An LTI system has unit impulse response $ h[n]=u[-n] $ Compute the system's response to the input $ x[n]=2^{n}u[-n]. $(Simplify your answer until all \sum signs disappear.)
solution
$ y[n]=x[n]*h[n] $
$ y[n]=\sum^{\infty}_{k=-\infty}x[n]*h[n-k] $
$ y[n]=\sum^{\infty}_{k=-\infty}2^{k}u[-k]u[n-k] $
$ n[-k] = 1 -k>=0, k<=0 $
$ y[n]=\sum^{0}_{k=-\infty}2^{k}*u[n-k] $
$ \sum^{0}_{k=n}, n<=0 $
$ 0, else $
