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Please help solve simultaneous inverse trig equations

  1. May 3, 2006 #1
    A friend of mine solved these pair of inverse trig equations simultaneously, claiming he did it with high school maths.

    Well, I have a similar background ( & am a keen puzzle solver ) & could get nowhere with them !

    Here they are :

    ArcTan (8/x) = ArcCos ((x+y)/20)

    ArcTan (8/y) = ArcCos ((x+y)/30)

    Solve these inverse trigonometric equations for x & y

    ( I will be extrememely embarrased if the method is trivial, so please forgive me if i've wasted your time :confused: )
     
  2. jcsd
  3. May 3, 2006 #2
    ArcTan (8/x) = ArcCos ((x+y)/20)

    ArcTan (8/y) = ArcCos ((x+y)/30)

    OK.... here goes.
    arctan x = i/2 log ((1-ix)/(1+ix))
    And..
    arc cos x = -i log(x + sqrt(x^2 - 1))
    So, going back to the first equation...

    i/2 log ((1 - 8i/x)/(1 + 8i/x)) = -i log ( (x+y)/20 + sqrt ((x+y)^2/20^2 - 1)
    And the second equation...
    i/2 log ((1 - 8i/y)/(1 + 8i/y)) = -i log ( (x+y)/30 + sqrt ((x+y)^2/30^2 - 1)

    The Is cancel out, and using the properties of logarithms (namingly if log x = log y, x = y and k log x = log (x^k), you'll end up with a nonlinear system of two equations to solve... doesn't look like it'll be very pretty. ;)

    BTW, there's probably some relation between arctan and arccos or something if your friend could do it in high school-- either that or he/she's taken (or somehow learnt) complex math in school.
     
  4. May 3, 2006 #3
    are you sure you can go further from that ?
     
  5. May 3, 2006 #4
    Yup:
    i/2 log ((1 - 8i/x)/(1 + 8i/x)) = -i log ( (x+y)/20 + sqrt ((x+y)^2/20^2 - 1)
    And the second equation...
    i/2 log ((1 - 8i/y)/(1 + 8i/y)) = -i log ( (x+y)/30 + sqrt ((x+y)^2/30^2 - 1)
    Cancelling the Is...

    1/2 log ((1 - 8i/x)/(1 + 8i/x)) = -1 log ( (x+y)/20 + sqrt ((x+y)^2/20^2 - 1)
    And the second equation...
    1/2 log ((1 - 8i/y)/(1 + 8i/y)) = -1 log ( (x+y)/30 + sqrt ((x+y)^2/30^2 - 1)

    Property of logarithms-> k log x = log(x^k):
    log [[(1 - 8i/x)/(1 + 8i/x)]^1/2] = log ( [(x+y)/20 + sqrt ((x+y)^2/20^2 - 1)]^-1 )
    And the second equation...
    log ([(1 - 8i/y)/(1 + 8i/y)]^1/2) = log ([ (x+y)/30 + sqrt ((x+y)^2/30^2 - 1)]^-1 )

    Property log x = log y -> x = y:
    [(1 - 8i/x)/(1 + 8i/x)]^1/2 = [(x+y)/20 + sqrt ((x+y)^2/20^2 - 1)]^-1
    And the second equation...
    [(1 - 8i/y)/(1 + 8i/y)]^1/2 = [(x+y)/30 + sqrt ((x+y)^2/30^2 - 1)]^-1

    And from there (writing it down on a piece of paper makes it look a whole lot easier BTW) either use a numerical method (e.g. Newton-Raphson) to solve it or whatever seems appropriate...
    It might even reduce to a linear equation after a lot of simplification, who knows? :\
     
  6. May 3, 2006 #5

    dav2008

    User Avatar
    Gold Member

    I tried the approach of treating the angle in each equation as an angle in a triangle.

    That didn't get me very far though. I just got x=0=y.
     
  7. May 4, 2006 #6
    This was the method i tried & arc cos ((x + y )) 20, implies a triangle with sides : ( x + y ) , ( (400 - ( x + y )^2 ) ^ 1/2 ) , 20

    from which you can get an arc tan & equate it to 8 / x

    ( i did the same for the 2nd equation )

    You therefore end up with an equations with no trig, but polynomials in x, y & ( x + y )

    Unfortunately this reduced down to a quartic :confused:
     
    Last edited: May 4, 2006
  8. May 4, 2006 #7
    I'll give the context of those equations :

    It comes from a classic ladder problem in a puzzle book ( it's among the toughest of 500 puzzles ) :

    http://www.amazon.com/gp/product/04...002-3261868-4914459?s=books&v=glance&n=283155

    it's problem number 400 & states :

    "2 ladders 20 & 30 feet long, lean in opposite directions across a passageway. They cross at a point 8 feet above the floor. How wide is the passage ?"

    A friend of mine did a diagram, from which the original equations were derived - i hope this link works :

    http://i2.tinypic.com/x3h1mv.jpg

    anyhows, try an method you like ( the above is just one method of attack )
     
  9. May 4, 2006 #8

    VietDao29

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    Homework Helper

    Uhmm, I doubt that your friend is telling the truth. This problem looks horrible.
    Now arccos(x) is always equal to or greater than 0, right?, since: [tex]0 \leq \arccos (x) \leq \pi[/tex], so that means:
    [tex]\arctan \left( \frac{8}{x} \right) = \arccos \left( \frac{x + y}{20} \right) \geq 0[/tex], hence x > 0, since [tex]\arctan \left( \frac{8}{x} \right) \geq 0[/tex].
    Do the same for the second equation to obtain y > 0.
    Let:
    [tex]\arctan \left( \frac{8}{x} \right) = \arccos \left( \frac{x + y}{20} \right) = \alpha[/tex]. That means:
    [tex]\tan \alpha = \frac{8}{x} \Leftrightarrow 1 + \tan ^ 2 \alpha = \frac{x ^ 2 + 64}{x ^ 2} \Leftrightarrow \frac{1}{\cos ^ 2 \alpha} = \frac{x ^ 2 + 64}{x ^ 2} \Leftrightarrow \cos ^ 2 \alpha = \frac{x ^ 2}{x ^ 2 + 64}[/tex]
    And from [tex]\arccos \left( \frac{x + y}{20} \right) = \alpha[/tex], you'll have: [tex]\cos \alpha = \frac{x + y}{20}[/tex], so we have:
    [tex]\frac{(x + y) ^ 2}{400} = \frac{x ^ 2}{x ^ 2 + 64} \quad (1)[/tex].
    Do the same for the second equation to obtain:
    [tex]\frac{(x + y) ^ 2}{900} = \frac{y ^ 2}{y ^ 2 + 64} \quad (2)[/tex]
    Eleminate (x + y)2 from 2 equations above (Equation (1), and equation (2)) to abtain:
    [tex]\frac{x ^ 2}{x ^ 2 + 64} = \frac{9}{4} \times \frac{y ^ 2}{y ^ 2 + 64}[/tex]
    Solve for y in terms of x, we have:
    [tex]y ^ 2 = \frac{256 x ^ 2}{5 x ^ 2 + 576} \quad (3)[/tex]
    Substitute (3) into (1), we have:
    [tex]\frac{x ^ 2 + \frac{32 x ^ 2}{\sqrt{5x ^ 2 + 576}} + \frac{256 x ^ 2}{5 x ^ 2 + 576}}{400} = \frac{x ^ 2}{x ^ 2 + 64}[/tex]
    [tex]\Leftrightarrow \left( x ^ 2 + \frac{32 x ^ 2}{\sqrt{5x ^ 2 + 576}} + \frac{256 x ^ 2}{5 x ^ 2 + 576} \right) (x ^ 2 + 64) = 400 x ^ 2[/tex]
    [tex]\Leftrightarrow ( x ^ 2 (5 x ^ 2 + 576) + 32 x ^ 2 \sqrt{5x ^ 2 + 576} + 256 x ^ 2 ) (x ^ 2 + 64) = 400 x ^ 2 (5 x ^ 2 + 576)[/tex]
    [tex]\Leftrightarrow ( 5 x ^ 2 + 576 + 32 \sqrt{5x ^ 2 + 576} + 256 ) (x ^ 2 + 64) = 400 (5 x ^ 2 + 576)[/tex]
    [tex]\Leftrightarrow (32 \sqrt{5x ^ 2 + 576}(x ^ 2 + 64) = 400 (5 x ^ 2 + 576) - (5x ^ 2 + 832) (x ^ 2 + 64)[/tex]
    From there, square both sides, you'll have a quartic equation in x2 :yuck:
    Something like this:
    [tex]25 x^8-13600 x^6-2297600 x^4+203980800 x^2+28966912000 = 0[/tex]
    And there are 2 real roots:
    [tex]\left[ \begin{array}{l} x ^ 2 \approx 122.6338468 \\ x ^ 2 \approx 660.42895867809 \end{array} \right. \Rightarrow \left[ \begin{array}{l} x \approx 11.074 \\ x \approx 25.6988 \end{array} \right.[/tex]
    The second x is not valid since [tex]\frac{x + y}{20} \approx \frac{25.6988 + y}{20} > 1[/tex]
    From the first x, we have:
    [tex]y = \sqrt {\frac{256 x ^ 2}{5 x ^ 2 + 576}} = \frac{16 x}{\sqrt{5x ^ 2 + 576}} \approx 5.138[/tex]
    So the solution is:
    [tex]\left\{ \begin{array}{l} x \approx 11.074 \\ y \approx 5.138 \end{array} \right.[/tex]
     
    Last edited: May 4, 2006
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