What is meant by input and output resistance of a transistor amplifier in CE mode?

Explanation

i. The dynamic or ac input resistance is defined as the ratio of change in base-emitter voltage to the change in base current at a constant collector-emitter voltage.
i.e., `" " r_( i) = ((Delta V_("BE"))/(Delta I_(B)))_(V_("CE")`
ii The dynamic or ac output resistance is defined as the ratio of change in collector emitter voltage to the change in collector current at a constant base current.
i.e., `" " r_(0) = ((Delta V_("CE"))/(Delta I_(C) ))_(I_(B))`.