Re: [理工] [工數]拉式轉換
※ 引述《harrypotter2 (智囧)》之銘言:
: ※ 引述《tony6401 (慢哥)》之銘言:
: 以下為補習班解法,本人複變爛到不行,如有問題還請高手解釋
: (至於快不快就見仁見智了...)
: -1 N zt
: L {F(s)}=Σ Res F(z)e
: k=1 z→ak
: 4 4 iπ/4 i3π/4 i5π/4 i7π/4
: S +4a =0 →S=(√2a)e ,(√2a)e ,(√2a)e ,(√2a)e
: z^3 zt (z-ak)z^3 zt
: Res -------e =lim ------e
: z→ak z^4+4a^4 z→ak z^4+4a^4
: 1 3 zt
: =lim---z *e
: 4z^3
: 1 t*ak
: =--e
: 4
: -1 1 t(a+ia) t(a-ia) t(-a-ia) t(-a+ia)
: 所以L {F(s)}=--[e +e +e +e ]
: 4
: 1 at iat -iat 1 -at iat -iat
: =--e (e +e )+--e (e +e )
: 4 4
: e^(at)+e^(-at)
: =cosat*(--------)
: 2
: =cos(at)*cosh(at)
看到tony6401拆成那樣,使我有些靈感
3 3
s s
原式 = ----------------------- = -----------------
4 2 2 4 2 2 2 2 2
s + 4a s + 4a -4a s (s +2a )-(2as)
3
s 1 s-a 1 s+a
= --------------------------- = --- ------------ + --- -----------
2 2 2 2 2 2 2 2 2 2
(s -2as+2a )(s +2as+2a ) s -2as+2a s +2as+2a
1 s-a 1 s+a
= --- ----------- + --- -----------
2 2 2 2 2 2
(s-a) +a (s+a) +a
-1 -1 1 s-a 1 s+a
f(t) = L (s) = L [ --- ----------- + --- -----------
2 2 2 2 2 2
(s-a) +a (s+a) +a
1 at 1 -at
= ---e cosat + ---e cosat
2 2
= coshatcosat
感覺留數比較直觀,還要感謝 tony6401 啟發靈感
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