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Homework Help: Finding θ Without the Aid of a Calculator

  1. Jul 18, 2012 #1
    1. The problem statement, all variables and given/known data

    These are three separate problems

    sec θ = 2

    cot θ =1

    csc θ = 2

    2. Relevant equations

    I'm not sure which of the trigonometric identities to use here. Therefore, I'm not sure which equations are relevant.

    3. The attempt at a solution

    I've never learned how to evaluate these without a calculator. I don't even know where to start. I tried using the 1 + tan2θ = sec2θ. But I couldn't yield many results with this.
    Last edited: Jul 18, 2012
  2. jcsd
  3. Jul 18, 2012 #2


    User Avatar

    Staff: Mentor

    Welcome to the PF.

    Start by writing them in the form of sin, cos, tan. Those functions are defined simply in terms of them...

    Then draw the triangles associated with each of them...
  4. Jul 18, 2012 #3
    If secθ = 2, what is cosθ?
    If cotθ = 1, what is tanθ?
    If cscθ = 2, what is sinθ?

    Also, the equation in boldface in your posting is incorrect.
  5. Jul 18, 2012 #4


    Staff: Mentor

    As well as ambiguous. To reduce the ambiguity of expressions with the square of a trig function, use parentheses, like this: sin^2(θ) or better yet, sin2(θ).

    You can write exponents like this by clicking the Go Advanced button (which opens an advanced menu), and clicking the X2 button.
  6. Jul 18, 2012 #5
    The farthest I'm able to get is that

    1/cos θ = 2

    1/tan θ = 1

    1/sin θ = 2

    That was a typo. I just corrected it. I was actually using 1 + tan2 θ = sec2 θ

    In any case, I've gotten to the point where I'm at:

    cosθ = 2

    tanθ = 1

    sinθ = 2

    I'm still not sure how to proceed from here without a calculator. Normally, I would just input θ = cos-1(2) in my calculator, but I can't do that here.

    My apologizes. I didn't take the time to look at the advanced menu when I posted.
  7. Jul 18, 2012 #6


    Staff: Mentor

    These are fine.
    These are wrong.

    From the first set of equations, if 1/cosθ = 2, then cos θ = 1/2, and similarly for the other two.

    There are only a handful of angles whose cosines you are expected to know exactly (i.e., without using a calculator), but this is one of them.
    Last edited: Jul 19, 2012
  8. Jul 18, 2012 #7

    cosθ = 1/2
    tanθ = 1
    sinθ = 1/2

    I still don't know exactly what it is yet. Is there a way of figuring it out or will I just have to memorize a small list of common solutions?
    Last edited by a moderator: Jul 19, 2012
  9. Jul 18, 2012 #8
    draw 2 right right triangles: one with 45°,45° and 90°, the other with 30°,60°,90°. Observe the sin,cos,tan. You will find out what's going on quickly.
  10. Jul 18, 2012 #9
    Ohhhh I see now.

    a. θ = 60º
    b. θ = 45º
    c. θ = 30º

    Wow, it's a lot simpler than I thought it would be. I'm kicking myself right now. Anyway, thanks for the help. My textbook wasn't very clear.
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