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Homework Help: Are these trig identities correct?

  1. Oct 26, 2007 #1
    cos(90+theta) = sin(theta)

    sin(90+theta) = -cos(theta)

    cos is negative in 2nd quad

    sin is positive in 2nd quad

    i'm looking at some soln's and i just need clarification, thanks!
    Last edited: Oct 26, 2007
  2. jcsd
  3. Oct 26, 2007 #2


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    Refer to the unit circle and you can answer this yourself.
  4. Oct 26, 2007 #3


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    No, they are not. Do you know an expression for cos(x+90) and sin(x+90), or more generally cos(a+b) and sin(a+b)?
  5. Oct 26, 2007 #4
    lol ah! why didn't it click, thanks again cristo :-]

    let me verify these soln's i'm typing up by using the sums/differences or cofunction identities.
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