设$f(t)=sin(t-\frac{\pi}{3})$，则$L[f(t)]=?$
A. $\frac{1-\sqrt 3 s}{2(1+s^2)}$
B. $\frac{s-\sqrt 3}{2(1+s^2)}$
C. $\frac{1}{1+s^2}e^{-\frac{\pi}{3}s}$
D. $\frac{s}{1+s^2}e^{-\frac{\pi}{3}s}$
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函数$\frac{2e^{-s}-e^{-2s}}{s}$的Laplace逆变换$L^{-1}[\frac{2e^{-s}-e^{-2s}}{s}]=?$
A. $u(t-2)-2u(t-1)$
B. $u(t-1)-2u(t-2)$
C. $2u(t-1)-u(t-2)$
D. $2u(t-2)-u(t-1)$
信号$f(t)=e^{-t}u(t-2)$的拉氏变换为
A. $\frac{1}{s-2}e^{-s-1}$
B. $\frac{1}{s+1}e^{-2(s+1)}$
C. $\frac{1}{s-2}e^{-2(s-1)}$
D. $\frac{1}{s+1}e^{-(s+1)}$
某信号的Z变换结果是$X(z)=\frac{1-az^{-1}}{z^{-1}-a}(a不等于0）$，则$x[0]=?$
A. $a$
B. $-a$
C. $\frac{1}{a}$
D. $-\frac{1}{a}$
当$0.5
A. $(\frac{1}{2})^nu[n]+2^nu[-n-1]$
B. $(\frac{1}{2})^nu[-n-1]+2^nu[n]$
C. $2^nu[n]-(\frac{1}{2})^nu[-n-1]$
D. $2^nu[n]+(-\frac{1}{2})^nu[-n-1]$
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