方法二： K,=im【(S-S, N(S)1=lim s-5MS S→S D(S) S→S d D(S) N(S)+(S-S)N'(S) lim N(S) S→S D'(S) D'(S)|S=-5, f0=r[FS=∑Ke i= 二、共轭复数根 F(S)= N(S) N(S) K∠0IK∠-0 S2+aS+b(S+o)2+02 S+o-jo S+o+j@ f(t)=L'[F(S)]=2Ke cos(at+)方法二： [( ) ( )] ( ) lim [( ) ] lim ( ) ( ) ( ) ( ) ( ) ( ) lim ( ) ( ) j j j j j j j S S S S j S S S S d S S N S N S dS K S S D S d D S dS N S S S N S N S D S D S → → = → − = − =   + −  = =       1 1 ( ) [ ( )] j n S t j j f t L F S K e − = = = 二、共轭复数根 2 2 2 ( ) ( ) ( ) ( ) N S N S F S S aS b S   = = + + + + K K 1 1 S j S j         − = + + − + + ( ) 1 1 ( ) [ ( )] 2 cos t f t L F S K e t    − − = = +