Principle of superposition.
Every signal is a superposition of unit impulse signals.
For an LTI system, the response to the unit impulse signal determines the response to any signal.
Definition and examples of convolution sum and convolution integral.
For an LTI system, the response to a signal is the convolution of that signal with the response to the unit impulse signal.

Feb 28 in class is suggested for the 1st midterm.