Please help me! Thanks a lot!
how to prove that y=x((cos(t))^2 is not Time Invariant?
Started by ●October 17, 2005
Reply by ●October 17, 20052005-10-17
novellover wrote:> Please help me! Thanks a lot!Start by proving that cos(t) is not time-invariant. Which of course would mean you knew the definition of time-invariant, cause you went to that lecture.
Reply by ●October 17, 20052005-10-17
Homework Problem? A system is time invarient IFF input x(t) results in the ouput y(t) and x(t-t0) results in the output y(t-t0). In other words, a shift in the input results in a corresponding shift in the output. To prove that the system is not time invarient, substitue in t-t0 for x, crank through the algebra, and show that it doesn't equal y(t-t0).
Reply by ●October 17, 20052005-10-17
Ooops, it should be y(t)=x((cos(t))^2), by the way, I created this problem myself... So please never ask if it is homework problem. NEVER!!! Otherwise it would be considered as offending. My questions are all original. And also please don't give non-meaningful replies -- like the previous two guys did. Thank you very much!
Reply by ●October 17, 20052005-10-17
Non-meaningfull replies..... Well guess what, my "non-meaningfull" reply can be found in just about any signal processing book as the definition of how to prove wether or not a system is time invarient, which, at the risk of offending you, is exactly what you asked for! Perhaps you should get one.
Reply by ●October 17, 20052005-10-17
novellover wrote:> Ooops, it should be y(t)=x((cos(t))^2), > > by the way, I created this problem myself... > > So please never ask if it is homework problem. > > NEVER!!! Otherwise it would be considered as offending. > > My questions are all original. > > And also please don't give non-meaningful replies -- like the previous > two guys did. Thank you very much! >It's just about the time of year that the frat rats start realizing that they need to stop drinking as much and start doing homework, so if you post questions that sound exactly like homework then expect to get responses as if you want free help with your homework. If you're offended by our responses then feel free to ask your questions elsewhere. People who don't have thick skins shouldn't attempt to use USENET. We don't know you, you're using an obvious alias, you're posting questions that could come right off of a take-home midterm -- what basis do we have for thinking it _isn't_ homework? Noway2 actually gave you just exactly the right answer to prove that _any_ system is or is not time invariant, and one that shouldn't be hard to apply to yours. He's a bit terse but he's right on track -- if you don't understand it then reply to his post with your attempt to do the math, and we'll help you out. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Reply by ●October 17, 20052005-10-17
novellover wrote:> My questions are all original.So how do you know that the equation is not Time Invariant? I mean, you want to know how to demostrate it, but, since you invented the question, how do you know it is true? bye, -- piergiorgio
Reply by ●October 17, 20052005-10-17
Tim, You are right, it is mid-term season - where does the time go. That probably explains why in the past week or so we have been seeing an increase in the number of very "homework like" problems on what would be covered in the first few chapters of a signal processing course. Matt
Reply by ●October 17, 20052005-10-17
novellover wrote:> ... > by the way, I created this problem myself... > > So please never ask if it is homework problem. > > NEVER!!! Otherwise it would be considered as offending. > ...I think you owe the group an apology. I know, *from personal experience* that they are kind and gentle to the ignorant. Don't believe me? Google the group for term "newbie" in "subject" and/or "body". Better yet, find just about any thread I started. Plagiarizing somebody's FAQ on netiquette: Clearly state your problem. Describe how you attempted to solve. Describe where your attempt failed. *POLITELY* ask for assistance -- 'please' and 'thank you' go a long way. [ I'm probably old enough to be your grandparent and have to admit to only recently learning the last :[
Reply by ●October 17, 20052005-10-17
novellover wrote:> Ooops, it should be y(t)=x((cos(t))^2), > > by the way, I created this problem myself... > > So please never ask if it is homework problem. > > NEVER!!! Otherwise it would be considered as offending. > > My questions are all original. > > And also please don't give non-meaningful replies -- like the previous > two guys did. Thank you very much!Buy a text book on the subject, or at least show to us that you have some idea of the concepts behind the question you're asking. You provided no attempt at solving the problem yourself, you didn't even indicate the direction you considered taking. You didn't show where you believe you lack understanding on the time-invariance topic. I'd say if anything you got a much more pleasant response than you deserved. Furthermore, telling us to NEVER!!! ask if it's a homework question is just plain rude, and quite schoolboyish. If we believe it's a homework problem, then we'll ask, or we'll just behave as if it is one. After all, it's the original poster that's wanting the help, so it's their responsibility to prove they aren't just after it for use in an assignment. We don't want to be any part of plagiarism.
