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Homework Help: Y=cosx is decreasing

  1. Mar 30, 2008 #1
    Hmm ... I did this problem for a friend.

    From what x values is [tex]y=cos x[/tex] decreasing from [tex]-\pi\leq x\leq\pi[/tex]

    http://img440.imageshack.us/img440/3876/baocu5.jpg [Broken]​
    *it should be [tex]-\frac{\pi}{2} \ \mbox{not} \ -\frac{3\pi}{2}[/tex]

    It's decreasing from [tex](0,\pi)[/tex] but the answer she gave me from the back of the book is [tex]-\frac{3\pi}{2}<x<\frac{3\pi}{2}[/tex]
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Mar 30, 2008 #2
    Nvm ... we apparently moved onto a different question, lol.
  4. Mar 30, 2008 #3
    The book's wrong
  5. Mar 30, 2008 #4
    Must be some of that new math :confused:
  6. Mar 31, 2008 #5
    LOL, no she started asking about a different question ... I just didn't realize, ha. I was like wtf ... b/c the answer she gave me wasn't even in the interval they were asking about! haha, I kept saying no it's wrong! And I'll prove it by asking the PF Gods :)
  7. Mar 31, 2008 #6
    It looks a bit wierd when it askes for the answer on the interval [-pi,pi] the books gives the answer as [-1.5pi,1.5pi]
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