Results 1 to 2 of 2

Math Help - COMPASS trigonometry question

  1. #1
    Newbie
    Joined
    Jul 2008
    Posts
    6

    COMPASS trigonometry question

    Hi, I'm taking the a Math placement test this Saturday so I've been practicing for that, but here's a problem I just can't figure out:

    If sin(x+y) = sinx cosx + cosx siny
    find sin((pi/2)+αlpha).
    The answer to this is supposed to be cos aplha, but I'm not sure how you get there.

    I found a similar problem in one of my Math books so I substituted
    ((pi/2)+alpha) for x and y in the equation above. I got

    sin((pi/2)+alpha) = sin(pi/2) cos alpha + cos (pi/2) sin alpha

    then I went on to divide by sin(pi/2) and sin alpha

    (sin(pi/2) + sin alpha)/(sin alpha + sin(pi/2)) = cos alpha + cos(pi/2)

    the left side will cancel, right? So we have

    1 = cos alpha + cos (pi/2)

    pi/2 = 90degrees and cos 90degrees = 0, right?

    so then

    1 = cos alpha

    Is that correct? what about the 1 on the left side? Can I just leave it there?
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Bar0n janvdl's Avatar
    Joined
    Apr 2007
    From
    Meh
    Posts
    1,630
    Thanks
    6
    Quote Originally Posted by tami-chan87 View Post
    Hi, I'm taking the a Math placement test this Saturday so I've been practicing for that, but here's a problem I just can't figure out:

    If sin(x+y) = sinx cosx + cosx siny
    find sin((pi/2)+αlpha).
    The answer to this is supposed to be cos aplha, but I'm not sure how you get there.

    I found a similar problem in one of my Math books so I substituted
    ((pi/2)+alpha) for x and y in the equation above. I got

    sin((pi/2)+alpha) = sin(pi/2) cos alpha + cos (pi/2) sin alpha

    then I went on to divide by sin(pi/2) and sin alpha

    (sin(pi/2) + sin alpha)/(sin alpha + sin(pi/2)) = cos alpha + cos(pi/2)

    the left side will cancel, right? So we have

    1 = cos alpha + cos (pi/2)

    pi/2 = 90degrees and cos 90degrees = 0, right?

    so then

    1 = cos alpha

    Is that correct? what about the 1 on the left side? Can I just leave it there?
    No you made you mistake when you divided by (sin(pi/2) + sin(alpha))

    (Besides that, your division is completely wrong.)

    sin((pi/2)+alpha) = sin(pi/2) cos alpha + cos (pi/2) sin alpha

    sin((pi/2)+alpha) = (1) cos alpha + (0) sin alpha = cos (alpha)
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. compass question
    Posted in the Geometry Forum
    Replies: 2
    Last Post: February 12th 2010, 10:45 AM
  2. Suspended compass
    Posted in the Geometry Forum
    Replies: 1
    Last Post: January 1st 2010, 03:45 AM
  3. Compass Placement Test
    Posted in the Algebra Forum
    Replies: 1
    Last Post: January 5th 2009, 04:22 PM
  4. compass directions
    Posted in the Trigonometry Forum
    Replies: 1
    Last Post: September 3rd 2008, 09:31 AM
  5. COMPASS trigonometry question
    Posted in the Trigonometry Forum
    Replies: 3
    Last Post: July 31st 2008, 08:12 AM

Search Tags


/mathhelpforum @mathhelpforum