Results 1 to 10 of 10

Math Help - never learned how to change a cosine into frational form

  1. #1
    Member
    Joined
    Jan 2011
    Posts
    75

    never learned how to change a cosine into frational form

    I was wondering if someone could help me out with how to change a cosine into fractional from: example cos 45 = sqrt2/2 and cos 150 = sqrt3/2

    I know that if I take the inverse of the cosine decimal form I can take the answer do square it, make a fraction out of it with the sqrt of the square dived by the squared number. that gets really messy as well though.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Master Of Puppets
    pickslides's Avatar
    Joined
    Sep 2008
    From
    Melbourne
    Posts
    5,236
    Thanks
    28
    Are you saying...

    When solving x = \cos 45^o , how can you get a fractional answer rather than some nasty decimal?
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Member
    Joined
    Jan 2011
    Posts
    75
    yeah, I can do it the way I explained but with most angles, that will still achieve a nasty decimal
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Master Of Puppets
    pickslides's Avatar
    Joined
    Sep 2008
    From
    Melbourne
    Posts
    5,236
    Thanks
    28
    Only particular angles will give you something nice.

    In general these are 0,30,45,60,90 and multiples of these. You can achieve a rational answer for others, but start by working on these ones first.

    Have you seen the special triangles?
    Follow Math Help Forum on Facebook and Google+

  5. #5
    Member
    Joined
    Jan 2011
    Posts
    75
    I haven't seen the special triangles.
    Follow Math Help Forum on Facebook and Google+

  6. #6
    Master Of Puppets
    pickslides's Avatar
    Joined
    Sep 2008
    From
    Melbourne
    Posts
    5,236
    Thanks
    28
    They are my favourite triangles. Commit them to memory and you will be a trig-wiz!

    Special Triangles
    Follow Math Help Forum on Facebook and Google+

  7. #7
    -1
    e^(i*pi)'s Avatar
    Joined
    Feb 2009
    From
    West Midlands, England
    Posts
    3,053
    Thanks
    1
    Essentially your construct a triangle with angles 60-30-90 and 45-45-90

    Special right triangles - Wikipedia, the free encyclopedia
    Follow Math Help Forum on Facebook and Google+

  8. #8
    Member
    Joined
    Jan 2011
    Posts
    75
    awesome, thanks a bunch
    Follow Math Help Forum on Facebook and Google+

  9. #9
    MHF Contributor
    Joined
    Mar 2010
    From
    Florida
    Posts
    3,093
    Thanks
    5
    You can also google the unit circle.
    Follow Math Help Forum on Facebook and Google+

  10. #10
    Senior Member
    Joined
    Nov 2010
    From
    Staten Island, NY
    Posts
    451
    Thanks
    2
    A side note: Most of those decimal answers you are getting are not exact answers. They are only approximations (although good approximations) to the actual answers. The actual answers are irrational and therefore cannot be written as finite decimals.
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. The book that Ramanujan learned from
    Posted in the Math Forum
    Replies: 0
    Last Post: March 5th 2011, 05:35 AM
  2. Change the form to y=mx+c
    Posted in the Math Topics Forum
    Replies: 6
    Last Post: March 3rd 2010, 12:39 AM
  3. Do I need to change this into parametric form?
    Posted in the Calculus Forum
    Replies: 2
    Last Post: November 21st 2009, 04:34 AM
  4. change to the form Acos(x+w)
    Posted in the Trigonometry Forum
    Replies: 1
    Last Post: November 3rd 2009, 10:07 AM
  5. HELP! Change to rectangular form!
    Posted in the Pre-Calculus Forum
    Replies: 1
    Last Post: May 28th 2008, 06:59 PM

Search Tags


/mathhelpforum @mathhelpforum