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: Trignometric Identities Problem

  1. Jan 29, 2005 #1
    A little confused on how to begin the problem

    _1__ - tanx sinx = cosx

    I know you change tanx to sinx/cosx but I cant seem to finish the problem, not sure if it is arithmatic errors or what?
    Last edited: Jan 29, 2005
  2. jcsd
  3. Jan 29, 2005 #2
    Just simplify both sides, by taking the most logical step... And remember [itex]cos^2 x +sin^2 x=1[/itex]
  4. Jan 29, 2005 #3


    User Avatar
    Science Advisor
    Homework Helper

    Write everything uder the same denominator.And the definition of tangent and the sine^{2}+cos^{2} connection.

  5. Jan 29, 2005 #4
    You had the right idea starting with [tex] tanx = \frac{sinx}{cosx} [/tex]

    Are you familiar with the trig identity: [tex] sin^2 x + cos^2 x = 1 [/tex]. In many trig question, you have to multiply/divide out parts of your equation to work out the final solution. Take a look at the question and see if there are any common values that would be worth taking out.
  6. Jan 29, 2005 #5
    Alright so

    _1__ - tanx sinx = cosx
    _1__ - sinx sinx = cosx
    cosx cosx
    _1__ - sinx
    cosx cosx

    Or is it

    cosx-cosxsinx * sinx
    ` 1```` 1 ```````1
  7. Jan 29, 2005 #6
    Now find a common denominator and simplify it.
  8. Jan 29, 2005 #7


    User Avatar
    Science Advisor
    Homework Helper

    OMG,okay here goes
    [tex] \frac{1}{\cos x}-\frac{\sin x}{\cos x} \sin x=... [/tex]

    Can u take it from here?

  9. Jan 30, 2005 #8
    ok, i guess ill provide a little more help:
    [tex]\frac{1}{cos\theta}-tan\theta sin\theta=cos\theta[/tex]

    simplify the tan:

    multiply and subtract, because you have like denominators:

    now, use the fact that [itex]sin^2\theta + cos^2\theta = 1[/itex] to solve.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook