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Math Help - work out missing angle?

  1. #1
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    work out missing angle?

    hi, i need to work out the angle in this shape:
    work out missing angle?-shape1.jpg

    im not really sure that to do but would this work:
    cosA= 5^2 + 10^2 + 18^2/ 2x5x10= 449/100= 4.49?

    is this correct? if not could you please tell me where im going wrong and what i need to do!?!

    Thanks!
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  2. #2
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    I don't think you're setting up your law of cosines correctly. Take a look at the wiki link, and tell me what the equation ought to be.
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  3. #3
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    one question, where would A,B and C be on my triangle?
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  4. #4
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    Take a really good look at the wiki link. It explains how to assign all the variables in the equation there. Then you tell me how to assign them.
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  5. #5
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    would it be, ?^2= 7^2 + 5^2-2x7x5xcos38= 18.84, then square root it= 4.34?
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  6. #6
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    Am I looking at the same picture you're looking at? I see a triangle with lengths 11, 18, and 10. You're asked to find the angle opposite the side of length 18. Is that correct?
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  7. #7
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    yes, would it be:
    18^2= 7^2+5^2-2x5x7xcos?= ??
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  8. #8
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    Where is the 7 and 5 coming from in your equation?
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  9. #9
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    GRRRR!!!! really sorry i was looking at the wrong question!

    would it be:18^2= 10^2+11^2-2x10x11xcos?= ??
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  10. #10
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    That looks better. Try solving for the cosine of the angle. What do you get?
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  11. #11
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    im not really sure how i would do that.
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  12. #12
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    Well, if you need to, just substitute x = cos(?) and solve for x the way you would normally do in algebra. What do you get?
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  13. #13
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    i can't think im having a mental blank! grr!
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  14. #14
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    could you tell me how to do it please, i can't think!
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  15. #15
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    Yes, you can think. Your equation is this:

    18^{2}= 10^{2}+11^{2}-2\times 10\times 11\times \cos(?)

    Simplifying one step leads to

    324=100+121-220\cos(?)

    Your goal, as usual, is to get the \cos(?) on one side of the equation and everything else on the other. What's your next step?
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