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Math Help - trigonometric equation

  1. #1
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    trigonometric equation

    Hi, I have problem that asks, "Prove that the equations are identities."
    I tried for hours to solve it, but I'm lost... I would really appreciate it if someone could help me on this.

    cot A + tan A + 1 = (cot A)/(1-tan A) + (tan A)/(1-cot A)

    The stipulation is that I can only use the trigonometric identities that I've learned so far..

    Such as:
    csc a = 1/sin a
    sec a = 1/csc a
    tan a = sin a/cos a
    etc...

    The pythgorean identities and Opposite angle formulas

    So far I've tried switching it sines and cosines, tangent, and a few others but it never seems to come out right...

    Thanks to anyone who can help.
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  2. #2
    is up to his old tricks again! Jhevon's Avatar
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    Quote Originally Posted by mi986 View Post
    Hi, I have problem that asks, "Prove that the equations are identities."
    I tried for hours to solve it, but I'm lost... I would really appreciate it if someone could help me on this.

    cot A + tan A + 1 = (cot A)/(1-tan A) + (tan A)/(1-cot A)

    The stipulation is that I can only use the trigonometric identities that I've learned so far..

    Such as:
    csc a = 1/sin a
    sec a = 1/csc a
    tan a = sin a/cos a
    etc...

    The pythgorean identities and Opposite angle formulas

    So far I've tried switching it sines and cosines, tangent, and a few others but it never seems to come out right...

    Thanks to anyone who can help.
    start by adding the fractions on the right hand side and simplify. now do you see how to get it?
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  3. #3
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    That's one of the first things I did. :-s
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  4. #4
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    Hello, mi986!

    Here's one way . .


    \cot A + \tan A + 1 \:= \:\frac{\cot A}{1-\tan A} + \frac{\tan A}{1-\cot A}

    The right side is: . \frac{\frac{1}{\tan A}}{1 - \tan A} + \frac{\tan A}{1 - \frac{1}{\tan }}


    Multiply each fraction by \frac{\tan A}{\tan A}\!:\quad {\color{blue}\frac{\tan A}{\tan A}}\cdot\frac{\frac{1}{\tan A}}{1 - \tan A} + {\color{blue}\frac{\tan A}{\tan A}} \cdot\frac{\tan A}{1 - \frac{1}{\tan A}}

    . . =\; \frac{1}{\tan A(1 - \tan A)} + \frac{\tan^2A}{\tan A - 1} \;=\;\frac{1}{\tan A(1 - \tan A)} - \frac{\tan^2\!A}{1-\tan A}


    Multiply the second fraction by \frac{\tan A}{\tan A}\!:\quad\frac{1}{\tan A(1 - \tan A)} - \frac{\tan^3\!A}{\tan A(1 - \tan A)}

    . . and we have: . \frac{1-\tan^3\!A}{\tan (1 - \tan A)}\;\;^{\leftarrow\;\text{difference of cubes}}


    Factor: . \frac{(1-\tan A)(1 + \tan A + \tan^2\!A)}{\tan A(1 - \tan A)} \;=\;\frac{1 + \tan A + \tan^2\!A}{\tan A}

    . . = \;\frac{1}{\tan A} + \frac{\tan A}{\tan A} + \frac{\tan^2\!A}{\tan A} \;=\; \cot A + 1 + \tan A

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  5. #5
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    Ooh! Thank you so much!
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