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    Doglover2011's Avatar
    Doglover2011 Posts: 16, Reputation: 1
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    #1

    Oct 11, 2009, 02:35 PM
    How to finsh this Trig.
    I'm trying to study for a test, but I get stuck a lot.
    I'm doing a problem that says to find the exact value of Cos 15 using sum and difference formulas. Well, I'm almost done but got a little stuck at the last part. Here is what I have using Cos(45-30):
    (1/sqre. Root of 2)(sqr. Root of 3/2) + (sqre. Root of 2/2) (1/2)
    Which is (Do I have this right? I think so... ) sqre. Root of 3/2 times sqre. Root of 2 + sqre. Root of 2/4.

    Which equals 6/??
    Perito's Avatar
    Perito Posts: 3,139, Reputation: 150
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    #2

    Oct 11, 2009, 03:04 PM
    Quote Originally Posted by Doglover2011 View Post
    I'm trying to study for a test, but I get stuck alot.
    I'm doing a problem that says to find the exact value of Cos 15 using sum and difference formulas. Well, I'm almost done but got a little stuck at the last part. Here is what I have using Cos(45-30):
    (1/sqre. root of 2)(sqr. root of 3/2) + (sqre. root of 2/2) (1/2)
    Which is (Do I have this right? I think so...) sqre. root of 3/2 times sqre. root of 2 + sqre. root of 2/4.

    Which equals 6/????








    You can simplify it a bit because and you can combine them under a common denominator, but you will always have an irrational value.
    galactus's Avatar
    galactus Posts: 2,271, Reputation: 282
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    #3

    Oct 11, 2009, 03:05 PM
    Doglover2011's Avatar
    Doglover2011 Posts: 16, Reputation: 1
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    #4

    Oct 11, 2009, 03:14 PM

    Galactus- Shouldn't 1/sqre root of to be sqre root of 2/2?
    galactus's Avatar
    galactus Posts: 2,271, Reputation: 282
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    #5

    Oct 11, 2009, 03:58 PM

    That's the same thing.
    Unknown008's Avatar
    Unknown008 Posts: 8,076, Reputation: 723
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    #6

    Oct 14, 2009, 10:33 AM

    Hey hey hey!! :mad:





    Then;



    Then, you can go on rationalising if you want:


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