1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: How do i find the range of this function?

  1. May 14, 2012 #1
    1. The problem statement, all variables and given/known data
    Find the range of y = (cos2x - 1) / ( cos2x + cos x )

    2. Relevant equations

    3. The attempt at a solution
    Well i tried the usual method. I cross multiplied and got a quadratic equation in cos x. Then it should have discriminant greater than zero so in the end i get (y-2)2 > 0 which is true for all y. So range is R? ( I don't have the answer)
  2. jcsd
  3. May 14, 2012 #2


    User Avatar
    Science Advisor

    Is the formula given correct? Do you really mean
    [tex]y= \frac{cos(x)-1}{2cos(x)}[/tex]?
    That is equal to
    [tex]\frac{1}{2}- \frac{1}{2cos(x)}[/tex]
    Since 2cos(x) is never larger than 2 or less than -2, 1/(2cos(x)) is always larger than 1/2 or less than -1/2, never between.
  4. May 14, 2012 #3


    User Avatar
    Homework Helper

    Try factoring the equation, you should get a nice simplification.

    But when you simplify you need to keep in mind that if you have an expression of the form [tex]y=\frac{ab}{b}[/tex] then when you simplify to y=a, you need to keep in mind that [itex]b\neq 0[/itex], so you would have to exclude this value if it's in the range.
  5. May 14, 2012 #4
    The problem with your method is that since cos is a restricted function, its own range is only between [-1,1]. To use your method, the equation should be in tan or cot functions since they have range R. Or you can restrict the answer you get accordingly to the range of cos.

    An easier method would be to factorize the equation since it results in a very simple equation.

    Edit : just noticed Mentallic already posted the easier way.
  6. May 14, 2012 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Simplify [itex]\displaystyle \frac{\cos^2(x)-1}{\cos^2(x)+\cos(x)}\ .[/itex]

    Factor the numerator & the denominator, then cancel, keeping in mind what Mentallic & Infinitum mentioned.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook