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Trigonometry Problem - Confused over multiple possible angle sizes.

  1. May 16, 2013 #1
    1. The problem statement, all variables and given/known data
    In triangle PQR, the length of QR is 4 mm, the length of PR is 3.5 mm and the size of angle Q is 60°. What are the possible size(s) of angle P (to 2 decimal places)
    The possible solutions are 138.81, 81.79, 118.96, 61.04, 98.21

    3. The attempt at a solution

    I've solved for 81.79 with the Sine Rule.
    Sine P/4 = Sine 60/3.5
    Sine P/4 = 0.247
    Sine P = 0.98
    Arcsin P = 81.79.

    What has me confused is where I can get multiple possible sizes of the angle. How can I detect when this is going to occur and how can I calculate all the possible angles? Thank you.
  2. jcsd
  3. May 16, 2013 #2


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    It's not clear what situations you think will result in multiple angles.

    For triangles, it is important to remember that the sum of the interior angles must equal 180 degrees.

    For this problem, angle Q is given as 60 degrees, so that would eliminate the first choice (138.81 degrees) automatically, since 60 + 138.81 > 180.
  4. May 16, 2013 #3
    It's a multiple choice that says select all possible angles, so there may be more than one answer and I've heard of this before.
    Here's another problem that's similar:
    In the triangle XYZ, the length of YZ is 15 cm, the length of XZ is 30 cm. If the size of angle Y is 20°, what are the possible size(s) of angle X (to 2 decimal places)?
    Solutions are 170, 10, 9.85, 30, 170.15
    I know one is 9.85 as I solved it with the sine rule, I just don't know where I'd get multiple values for the angle, which I'm sure does happen.
  5. May 16, 2013 #4


    Staff: Mentor

    Maybe draw a sketch of the triangle. From the sketch that I drew, I can see that the values given for QR and PQ and angle Q open up a couple of possibilities for the length of QP.
  6. May 16, 2013 #5
    I've drawn a sketch already to use the Sine Rule. Would you mind explaining how you can get multiple angles and how you can calculate them?
  7. May 16, 2013 #6


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    Gold Member

    Draw the triangle using (in principle, at least) ruler and compass construction. Start with the line QR. At Q you have an angle of 60 degrees, so you can draw a line from there, P lying somewhere on it. You know the distance of P from R. How would you construct the point P?
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