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Math Help - Rationalising surd....

  1. #1
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    Rationalising surd....

    okay, i seem to be pretty cool with the basics of rationalizing surds, just started doing a level maths, is going okay.

    three questions, though, i keep trying to do and can't see where i'm going wrong, here they are.

    don't know how to do the symbols so will just write it

    ********

    square root of 6,

    over

    square root of six, plus square root of 3

    i get 6 minus square root of 18,

    over,

    3

    however, the book says ....

    2 minus square root of 2,....

    i really can't see how they got this, hopefully if its explained, i'll be able to do the other two.

    thanks!
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  2. #2
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    Have you tried using factors of 18 under the square root sign?
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  3. #3
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    oh man i can't believe i missed that. thanks !
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  4. #4
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    had missed that on the other question as well, fixed that proper good.

    3rd question though, i still can't get;

    root 7 + 5

    over

    root 7 + root 5

    i get

    7 minus root 35, plus 5 times root 7, minus 5 times root 5

    over

    2

    where the hell am i going wrong!!
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  5. #5
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    But its not root 7 + root 5, its (root7) + 5.

    multiplying that by the rationalising factor (root 7 - root 5) gives;

    7, - root35, + 5(root7), - 5(root5)

    this book is known to have a few mistakes, by the way!
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  6. #6
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    Quote Originally Posted by artem1s View Post
    But its not root 7 + root 5, its (root7) + 5.

    multiplying that by the rationalising factor (root 7 - root 5) gives;

    7, - root35, + 5(root7), - 5(root5)

    this book is known to have a few mistakes, by the way!
    Hello, artem1s. This should clear it up for you.

    \frac{\sqrt{7}+5}{\sqrt{7}+\sqrt{5}}\cdot\frac{\sq  rt{7}-\sqrt{5}}{\sqrt{7}-\sqrt{5}} =\frac{7-\sqrt{35}+5\sqrt{7}-5\sqrt{5}}{7-5} = \frac{7-\sqrt{35}+5\sqrt{7}-5\sqrt{5}}{2}
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  7. #7
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    Yeah, that's what i got as well! Rotten book was wrong!

    Thanks guys, i was tearing my hair out trying to figure this one out.
    Last edited by mr fantastic; April 7th 2009 at 10:57 PM.
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