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For any <math>x_{1}(t) \; \rightarrow \; y_{1}(t)</math> and <math>x_{2}(t) \; \rightarrow \; y_{2}(t)</math> | For any <math>x_{1}(t) \; \rightarrow \; y_{1}(t)</math> and <math>x_{2}(t) \; \rightarrow \; y_{2}(t)</math> | ||
| − | <DIV CLASS="indented"> | + | <DIV CLASS="indented"> and any complex constants <math>a</math> and <math>b</math></DIV> |
| − | and any complex constants <math>a</math> and <math>b</math> | + | |
| − | </DIV> | + | |
then | then | ||
Revision as of 21:30, 16 September 2008
Problem
A linear system’s response to $ e^{2jt} $ is $ te^{-2jt} $, and its response to $ e^{-2jt} $ is $ te^{2jt} $. What is the system’s response to $ cos(2t) $?
Solution
If the system is linear, then the following is true:
For any $ x_{1}(t) \; \rightarrow \; y_{1}(t) $ and $ x_{2}(t) \; \rightarrow \; y_{2}(t) $
and any complex constants $ a $ and $ b $
then
$ ax_{1}(t) \; + \; bx_{2}(t) \; \rightarrow \; ay_{1}(t) \; + \; by_{2}(t) $
and "conveniently":
$ e^{2jt} \; + \; e^{-2jt} = \cos{(2t)} \; + \; j \sin{(2t)} \; + \; \cos{(-2t)} \; + \; j \sin{(-2t)} $ (by Euler's Formula)
$ =\cos{(2t)} \; + \; j \sin{(2t)} \; + \; \cos{(2t)} \; - \; j \sin{(2t)} $ ($ \cos{(-x)}=\cos{(x)} $ and $ \sin{(-x)}=-\sin{(x)} $)
$ =2\cos{(2t)} $
