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trig question

Discussion in 'DAT Discussions' started by msu08, Dec 27, 2008.

  1. msu08

    7+ Year Member

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    the value of cos (pi/3) equals the value of

    A. -cos 2pi/3
    B. cos 2pi/3
    C. cos 6pi/3
    D. -cos 5pi/3
    E. cos 4pi/3

    answer is A why????

    another ....

    for all y, the cos y is equal to which of the following?
    A. siny
    B. cos(y + pi)
    C. sin (-y)
    D. sin y+ cos y
    E. cos (-y)

    answer is E why??
    these problems really get me. i know they're not hard, it's just when i look at them, i'm not even sure about what trig info i should be using to solve them. any suggestions about which identities, etc. i should know for the QR section as well as explanations to these problems would be fantastic! thank youu!
     
  2. FutureInternist

    10+ Year Member

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    You have to memorise the unit circle for this, or atleast the first quadrant from which you can derive the rest of the values.

    For the first one...cos(pi/3) = 0.5 (i.e. a positive number). Cos of angles are equal when reflected across the x -axis i.e. Cos (pi/3) = Cos (5pi/3)
    C is out since that is 2pi & cos 2pi = 1
    D is out since cos (pi/3) has the SAME value as cos 5(pi/3) (not it's negative)
    E is out since cos (4pi/3) is in the 3rd quad & hence is negative.

    That leaves A or B. Again, 2pi/3 is in the 2nd quad & hence its negative so B is out, leaving A as the only option.

    For the 2nd Q, again you just have to know it. Taking 60 (pi/3) as an example....
    cos 60=0.5
    cos 60 = cos -60 = cos300 = 0.5
    This also goes back to the first Q in that cos values are the same when reflected across the X-axis.

    Hope this helps.
    Post any more Qs here.
     
  3. FutureInternist

    10+ Year Member

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    This might help.
     
    #3 FutureInternist, Dec 27, 2008
    Last edited: Dec 27, 2008

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