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Math Help - Trig Identites

  1. #1
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    Trig Identites

    Hey guys.

    In the following problem, I'm supposed to perform the addition and then use the fundamental identities to simplify.

    Also please be loose on the format of the problem. I'm not very familiar with LaTeX at this point in time.

    (1/(1 + cos x)) + (1/(1 - cos x))

    What I did, since they had different denominators, I multiplied every fraction with (1 + cos x)(1 - cos x), eliminating the fractions and leaving me with

    (1 - cos x) + (1 + cos x)

    Thing is though, the answer is 2csc^2x, and the only thing I can do to the problem as I have it now is cancel the cos x and leave me with 2, which is obviously wrong. How is my thinking flawed on this problem?

    I appreciate any and all help contributed.
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  2. #2
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    Hello, mathgeek777!

    Simplify: . \frac{1}{1 + \cos x} + \frac{1}{1 - \cos x}
    You can't "eliminate the denominators".

    We can get a common denominator, though . . .


    \frac{1}{1+\cos x}\cdot{\color{blue}\frac{1-\cos x}{1-\cos x}} + \frac{1}{1-\cos x}\cdot{\color{blue}\frac{1+\cos x}{1+\cos x}} \;\;=\; \;\frac{(1-\cos x) + (1 + \cos x)}{(1+\cos x)(1-\cos x)}

    . . = \;\;\frac{2}{1-\cos^2\!x} \;\;=\;\;\frac{2}{\sin^2\!x} \;\;=\;\;2\csc^2\!x

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  3. #3
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    Quote Originally Posted by mathgeek777 View Post
    Hey guys.

    In the following problem, I'm supposed to perform the addition and then use the fundamental identities to simplify.

    Also please be loose on the format of the problem. I'm not very familiar with LaTeX at this point in time.

    (1/(1 + cos x)) + (1/(1 - cos x))

    What I did, since they had different denominators, I multiplied every fraction with (1 + cos x)(1 - cos x), eliminating the fractions and leaving me with

    (1 - cos x) + (1 + cos x)

    Thing is though, the answer is 2csc^2x, and the only thing I can do to the problem as I have it now is cancel the cos x and leave me with 2, which is obviously wrong. How is my thinking flawed on this problem?

    I appreciate any and all help contributed.
    You are simplifying this as if it is an equation where you can multiply both sides by something and come up with an equivalent equation. But this isn't an equation so you can't do the multiplication.

    -Dan
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  4. #4
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    Quote Originally Posted by Soroban View Post
    Hello, mathgeek777!

    You can't "eliminate the denominators".

    We can get a common denominator, though . . .


    \frac{1}{1+\cos x}\cdot{\color{blue}\frac{1-\cos x}{1-\cos x}} + \frac{1}{1-\cos x}\cdot{\color{blue}\frac{1+\cos x}{1+\cos x}} \;\;=\; \;\frac{(1-\cos x) + (1 + \cos x)}{(1+\cos x)(1-\cos x)}

    . . = \;\;\frac{2}{1-\cos^2\!x} \;\;=\;\;\frac{2}{\sin^2\!x} \;\;=\;\;2\csc^2\!x


    ...Oh! I knew that. Thanks! I appreciate your assistance.

    I do have another question though. How did you go from \;\frac{(1-\cos x) + (1 + \cos x)}{(1+\cos x)(1-\cos x)} to \frac{2}{1-\cos^2\!x}

    EDIT: Forgive the brain fart, I just realized how you did that.
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