拉普拉斯变换部分作业
拉普拉斯变换部分作业
1、求下列函数的拉普拉斯变换的象函数
(1)e -5t sin 4t (2)t (cos ωt +sin ωt )
(3)⎰t (cos 4t +sin 3t )dt (4)⎰e -2t cos 3t +e -4t t 3d t 00t t ()
sin t e bt -e at
(5) a ,b 为实常数 (6) t t
答案:(1)4
p +52+16
⎡12p 2+2p ω⎤(2)-⎢2 -2222⎥p +ωp +ω⎣⎦1⎡12p 26p ++(3)⎢-222p ⎢p +16p +16p 2+9⎣4⎤ 2⎥⎥⎦(4)p p +2+92p +2+6
p p +4
(5)ln
(6)p -a p -b -arctan p 2
2、求下列拉普拉斯变换象函数的原函数 π
(1)1
p -2p -32 (2)p p -2p -32
(3)p +8p (4)p 2+4p +5p 2+a 22a >0 答案:(1)-e 2t -te 2t +e 3t
(2)-3e 2t -2te 2t +3e 3t
(3)37e -2t cos [t +arctan (-6)]
1t sin at 2a
3、解下列常微分方程 (4)
d 2y (t )3t ()-y t -1-e =02dt (1) dy 1y (0)=0=t =0dt 2
d 3y (t )d 2y (t )dy (t )-t ()+3+3+y t =6e dt dt 3dt 2
(2) 2dy d y y (0)==2=0dt t =0dt t =0
d 2y (t )dy (t )2t ()-2+y t =t e 2dt dt (3) dy y (0)==0dt t =0
71答案:(1)y (t )=-1+e -t +e 3t 88
(2)y (t )=t 3e -t
(3)y (t )=14t t e 12
4、求解下列微分方程组
⎧dx (t )-t ()()+5x t +2y t =e ⎪⎪dt (1)⎨
⎪dy (t )+2x (t )+2y (t )=0⎪⎩dt
(2)x (0)=y (0)=1⎧d 2z (t )-z (t )=0⎪x (t )+y (t )+2dt ⎪⎪d 2x (t )-x (t )+y (t )+z (t )=0⎨2dt ⎪⎪d 2y (t )-y (t )+z (t )=0⎪x (t )+2dt ⎩z (0)=0
dx (t )dy (t )dz (t )===0dt t =0dt t =0dt t =0
4-t 1-t 4-6t e +te -e 25525答案:(1) 2-t 2-t 2-6t y (t )=e -te -e 25525x (t )=
12x (t )=ch 2t +cos t 33
12(2) y (t )=ch 2t +cos t 33
22z (t )=-ch 2t +cos t 33)))