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Math Help - tangent of inverse trignometric function

  1. #1
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    tangent of inverse trignometric function

    a) express in term of X :
    \sin\left( 2 \tan^{-1}(x)\right)

    b) Find: \tan\left( \sin^{-1}(\frac {5}{8}) + \cos^{-1}(\frac {1}{6})\right)
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  2. #2
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    Hello, viet!

    a) Express in term of x: . \sin\left(2\tan^{-1}(x)\right)

    We will need the identity: . \sin2\theta \:=\:2\sin\theta\cos\theta

    Recall that an inverse trig value is an angle.

    Let \theta = \tan^{-1}(x)\quad\Rightarrow\quad\tan\theta = x

    Since \tan\theta = \frac{x}{1} = \frac{opp}{adj}, we have:  opp = x,\:adj = 1
    . . and Pythagorus gives us: hyp = \sqrt{x^2+1}
    Hence: . \sin\theta = \frac{x}{\sqrt{x^2+1}},\;\cos\theta = \frac{1}{\sqrt{x^2+1}}


    The problem becomes: . \sin(2\theta) \:=\:2\sin\theta\cos\theta

    . . . . . . =\:2\left(\frac{x}{\sqrt{x^2+1}}\right)\left(\frac  {1}{\sqrt{x^2+1}}\right) \;=\;\boxed{\frac{2x}{x^2+1}}



    b) Find: . \tan\left[\sin^{-1}\left(\frac {5}{8}\right) + \cos^{-1}\left(\frac {1}{6}\right)\right]

    We have: . \tan\left[\underbrace{\sin^{-1}\left(\frac{5}{8}\right)}_{\alpha} + \underbrace{\cos^{-1}\left(\frac{1}{6}\right)}_{\beta}\right]


    Then: . \sin\alpha = \frac{5}{8}\quad\Rightarrow\quad opp = 5,\:hyp = 8\quad\Rightarrow\quad adj = \sqrt{39}
    . . Hence: . \tan\alpha = \frac{5}{\sqrt{39}}

    And: . \cos\beta = \frac{1}{6}\quad\Rightarrow\quad adj = 1,\:hyp = 6\quad\Rightarrow\quad opp = \sqrt{35}
    . . Hence: . \tan\beta = \frac{\sqrt{35}}{1} = \sqrt{35}


    The problem becomes: . \tan(\alpha + \beta)\;=\;\frac{\tan\alpha + \tan\beta}{1 - \tan\alpha\tan\beta} \;=\; \frac{\frac{5}{\sqrt{39}} + \sqrt{35}}{1 - \frac{5}{\sqrt{39}}\cdot\sqrt{35}}

    Multiply top and bottom by \boxed{\sqrt{39}\!:\;\;\frac{5 + \sqrt{1365}}{\sqrt{39} - 5\sqrt{35}}}


    I hope I didn't make any stupid blunders . . .
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  3. #3
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    I used your method for part B on a similar problem and it is not working.

    numbers were 5/6 for the sin, 5/9 for the cos.

    ended with (((5/sqrt(11))+(sqrt(56)/5))/(1-(5/sqrt(11))*(sqrt(56)/5)))*sqrt(11)
    but it is wrong
    Last edited by thedoge; February 4th 2007 at 07:23 PM. Reason: typo
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  4. #4
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    Hello, thedoge!

    I used your method for part B on a similar problem and it is not working.

    Numbers were 5/6 for the sin, 5/9 for the cos.

    Ended with: (((5/sqrt(11))+(sqrt(56)/5))/(1-(5/sqrt(11))*(sqrt(56)/5)))*sqrt(11) .??

    but it is wrong

    Evidently, your preliminary work is correct.
    . . \tan\alpha = \frac{5}{\sqrt{11}},\;\;\tan\beta = \frac{\sqrt{56}}{5}


    Identity: . \tan(\alpha + \beta) \;=\:\frac{\tan\alpha + \tan\beta}{1 - \tan\alpha\cdot\tan\beta}

    We have: . \tan(\alpha + \beta) \;=\;\frac{\frac{5}{\sqrt{11}} + \frac{\sqrt{56}}{5}} {1 - \left(\frac{5}{\sqrt{11}}\right)\!\left(\frac{\sqr  t{56}}{5}\right)}


    Multiply top and bottom by 5\sqrt{11}:\;\;\frac{25 + \sqrt{56}\sqrt{11}}{5\sqrt{11} - 5\sqrt{56}}

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  5. #5
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    Ah ok. Thank you
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