Results 1 to 7 of 7

Math Help - Quite an easy trig identity question - need pointers

  1. #1
    Newbie
    Joined
    Nov 2010
    Posts
    2

    Quite an easy trig identity question - need pointers

    Hello,

    I am trying to do the following question. I've just done a chapter which involved trig identities but apparently this one involves coordinate geometry though I'm not sure.

    Using the identities in your notes, express the following in terms of the sines and cosines of x, y, 3x and 3y.
    i) sin(3x - y)
    ii) cos (x + 3y)

    I realise this is probably a silly question but I just don't know how to go about it.
    for the first one, I know that sin(theta) = -sin(-theta), but i'm not sure that is releavant. I also know that sin squared theta + cos squared theta = 1, but I don't know how to manipulate the equations I've been given or what they mean.

    Any help at all is appreciated, many thanks in advance.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor
    Joined
    Oct 2009
    Posts
    5,561
    Thanks
    785
    There are formulas for the sine and cosine of sums and differences: see Wikipedia and MathWorld. I would expect that these formulas should be derived or given in class. You can also Google, for example, for "sine sum" to find the derivations.
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Junior Member
    Joined
    Nov 2010
    Posts
    50
    Have you learnt about the addition formulae in your math class?
    sin(A-B) = sinAcosB - cosAsinB
    cos(A+B) = cosAcosB - sinAsinB.

    I am not sure what the question wants but it may be useful to you.
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Newbie
    Joined
    Nov 2010
    Posts
    2
    Quote Originally Posted by arccos View Post
    Have you learnt about the addition formulae in your math class?
    sin(A-B) = sinAcosB - cosAsinB
    cos(A+B) = cosAcosB - sinAsinB.

    I am not sure what the question wants but it may be useful to you.
    thank you, I'd forgotten about that. so, I can write:
    sin (3x - y) = sin3xcosy - cos3xsiny ?

    can I do anything else to it?
    Follow Math Help Forum on Facebook and Google+

  5. #5
    Senior Member Educated's Avatar
    Joined
    Aug 2010
    From
    New Zealand
    Posts
    433
    Thanks
    12
    Using the identities in your notes, express the following in terms of the sines and cosines of x, y, 3x and 3y.
    i) sin(3x - y)
    ii) cos (x + 3y)
    I think that's the simplest you can simplify it down to, and it has satisfied your question.
    Remember to use brackets to make it easier to interperet.

    sin(3x - y) = sin(3x) cos(y) - cos(3x) sin(y)

    You could use the triple angle formulas but that just makes it more complicated.
    Follow Math Help Forum on Facebook and Google+

  6. #6
    is up to his old tricks again! Jhevon's Avatar
    Joined
    Feb 2007
    From
    New York, USA
    Posts
    11,663
    Thanks
    3
    Quote Originally Posted by Educated View Post
    [snip]

    You could use the triple angle formulas but that just makes it more complicated.
    ...and unnecessary, as the problem makes it clear having the angles 3x and 3y is fine.
    Follow Math Help Forum on Facebook and Google+

  7. #7
    Junior Member
    Joined
    Nov 2010
    Posts
    50
    Quote Originally Posted by foggynotion View Post
    thank you, I'd forgotten about that. so, I can write:
    sin (3x - y) = sin3xcosy - cos3xsiny ?

    can I do anything else to it?
    That would be sufficient to fit within the parameters of the question.
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Easy trig question
    Posted in the Trigonometry Forum
    Replies: 3
    Last Post: October 28th 2010, 04:51 PM
  2. easy trig question
    Posted in the Trigonometry Forum
    Replies: 5
    Last Post: September 17th 2009, 10:04 AM
  3. Trig identity. Probably really easy
    Posted in the Trigonometry Forum
    Replies: 2
    Last Post: November 30th 2008, 06:12 PM
  4. Pretty easy trig. question
    Posted in the Calculus Forum
    Replies: 3
    Last Post: July 20th 2008, 09:22 PM
  5. really easy trig question
    Posted in the Trigonometry Forum
    Replies: 1
    Last Post: August 31st 2005, 04:37 AM

Search Tags


/mathhelpforum @mathhelpforum