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Trigonometric equations (finding angles)

  1. Dec 9, 2015 #1
    1. The problem statement, all variables and given/known data
    ok so my professor gave me this problem to solve, it goes like this :(I will also have a picture below)
    In the square (ABCD) is a point P which divides the side BC into 2 halves and point R which divides the side CD into 2 halves
    The angles at APB and ARB are the same.
    Calculate the angle x and the ratio of sides of the square 20151209_214726.jpg

    2. Relevant equations
    he gave me an answer x=arc tan (2*sqrt(2))
    and
    a:b=sqrt(2):1
    I got those answers but I also got a second one and I an wondering if you guys can tell me why the 2nd answer is not possible

    3. The attempt at a solution
    ok so first I started out by writting sin(x)
    sin(x)= a/sqrt(a2+(b/2)2) i got that from the ABP triangle
    sin(x)=a/sqrt(a2+b2/4)
    sin(x)=a/sqrt((4a2+b2)/4)
    sin(x)=2a/sqrt(4a2+b2)
    secondly I calculated the lenght of the side AR
    AR=sqrt(b2+a2/4)
    AR=sqrt(4b2+a2)/2
    thirdly I use the formula to calculate the area of the ARB triangle
    Area=AR2*sin(x)/2
    Area=(4b2+a2)*2a/(8*sqrt(4a2+b2))
    Area=(4b2a+a3)/(4*sqrt(4a2+b2)
    ok from now on I figured out that this are is equal to half of the area of a square
    a*b/2=(4b2a+a3)/(4*sqrt(4a2+b2)
    2*b*sqrt(4a2+b2)=4b2+a2
    here i squared both sides
    4b2*(4a2+b2)=16b4+8b2a2+a4
    16a2b2+4b4=16b4+8b2a2+a4
    0=12b4-8a2b2+a4
    here i created a new variable m=b2
    0=12m2-8m*a2+a4
    m1,2=(8a2+(or)-sqrt(64a4-48a4))724
    m1,2=(8a2+(or)-4[aSUP]2[/SUP])/24
    m1=a2/2
    m2=a2/6
    ok so
    b2=a2/2
    and
    b2=a2/6
    I won't check for a2/2 because I already know that is the right answer. I'm just wondering why this is not the right answer as well
    b2=a2/6
    so know I finally returned to my initial equation

    sin(x)=2a/sqrt(4a2+b2) and I put in b2
    sin(x)=2a/sqrt(4a2+a2/6)
    sin(x)=2a/sqrt(25a2/6)
    sin(x)=2a*sqrt(6)/5a
    sin(x)=2*sqrt(6)/5
    and if I draw this triangle i can figure out the 3 side
    Snapshot.jpg
    and then I can calculate the angles as

    tan(x)=(2*sqrt(6))/............x=arc tan (2*sqrt(6))
    and
    a:b=sqrt(6):1
    can somebody help me find my mistake or tell me why this is not a possible answer ?
    thanks
     
    Last edited: Dec 9, 2015
  2. jcsd
  3. Dec 9, 2015 #2

    Simon Bridge

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    I can't tell your mistake bc I'm having trouble following your reasoning.
    You check answers by drawing out the resulting rectangle and checking the angles etc make sense. This approach also helps you understand what happened.

    I'd have divided side AB in two, labelling the halfway point Q
    Then I'd use the tangent definition on the triangles ABP and RQB
    ... the tangent half-angle formula looks useful here.
     
  4. Dec 10, 2015 #3

    ehild

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    Your approach allows the angle ARB greater than 90°, as you work entirely with sines. But the angle APB has to be less than 90°. The tangent of half of the angle ARB is a/(2b) and a/(2b)<1 must hold, that is a/b<2. Working with the half-angle formula as Simon suggested you get only a=√2 b. If you assume a=√6 b, you get negative value for the tangent of angle ARB, although its sine is the same as that of APB.
    Draw also the triangle ABR.
     
  5. Dec 10, 2015 #4
    Thank you
    just to check if I really get it
    if I would allow that the tangent half angle at ARB is >1 than that would mean that the whole angle would be over 90° and since the picture only shows the posibility of the angle being less 90° the second solution is ruled out.?
    Another question if I may.
    What is the best way to check your solutions in problems like this ? is it to draw it out or do somethng else?
     
  6. Dec 10, 2015 #5

    ehild

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    Yes, correct.
    It is always necessary to investigate the possible range of the solution(s). Here it is clear that the angle should be less than 90°,so the half-angle is less than 45°. Also useful to draw it, but then it is not so evident that a solution is wrong.
     
  7. Dec 10, 2015 #6

    Simon Bridge

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    I get ##a/b = \sqrt {6}## via half-angle formula. <sniff> :)
     
  8. Dec 10, 2015 #7

    ehild

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    How?
    tan(x)=2a/b
    tan(x/2)=a/(2b)
    ##\tan(x)=\frac{2\tan(x/2)}{1-\tan^2(x/2)}##, well, this is rather the double-angle formula, but it gives a/b=√2.
     
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