df(t)/dt △ △ △t △ (b) (a)a rectangular pulse of unit area which approaches a unit impulse as△→>0 (b)A modified (YE)unit-step function. (c) The derivative of the modified unit step The unit impulse may be regarded as the time derivative of the unit step function. du(t) (t)= or()=o()dt(r>0) dt(a) A rectangular pulse of unit area which approaches a unit impulse as  → 0. (b) A modified(准) unit-step function. (c) The derivative of the modified unit step. The unit impulse may be regarded as the time derivative of the unit step function. dt du t t ( )  ( ) =  − =  t or u t t dt t 0 ( )  ( ) ( 0) −  2 1  2 1  1 (a) t f (t) −  2 1  2 1 1 (b) t f (t) −  2 1  2 1  1 (c) t df (t)/ dt