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    (Original post by Pineappleface317)
    Hey I'm actually OCR but I couldn't find an OCR C4 2015 thread but I'm guessing you guys do vectors??
    Could someone help me with this question:

    Find a unit vector that is perpendicular to both 4i + 10j - 3k and 5i - 9j + 7k

    All I have so far is:
    4a + 10b - 3c = 0
    and
    5a - 9b + 7c = 0

    (ai + bj + ck being my unknown unit vector)

    Thanks
    Hint: it says "unit" vector

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    (Original post by Pineappleface317)
    Hey I'm actually OCR but I couldn't find an OCR C4 2015 thread but I'm guessing you guys do vectors??
    Could someone help me with this question:

    Find a unit vector that is perpendicular to both 4i + 10j - 3k and 5i - 9j + 7k

    All I have so far is:
    4a + 10b - 3c = 0
    and
    5a - 9b + 7c = 0

    (ai + bj + ck being my unknown unit vector)

    Thanks
    Maybe use c=1 and solve


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    (Original post by aersh8)
    Hint: it says "unit" vector

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    A very poor hint - if it could be considered a hint at all.
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    (Original post by Medicjug)
    Maybe use c=1 and solve


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    Probably better to use c=0

    But c=1 would also give a perpendicular vector - just easier when using c=0
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    (Original post by Medicjug)
    Maybe use c=1 and solve


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    just cross them!!!!!!
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    (Original post by Medicjug)
    Maybe use c=1 and solve


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    So that's just because each component in i k and k is one unit long? So I could make anything one?
    Sorry for some reason I can't get my head around vectors ..
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    (Original post by Tow)
    Probably better to use c=0

    But c=1 would also give a perpendicular vector - just easier when using c=0
    How come you can use c=0 ?
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    (Original post by Pineappleface317)
    So that's just because each component in i k and k is one unit long? So I could make anything one?
    Sorry for some reason I can't get my head around vectors ..
    No, think of it this way if som vector is parralel to both it will be a mutiple of some direction vector. We can factor out a multiple such that c=1 leaving a,b not determined as of yet and then adjust the actual vector in the end.


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    (Original post by Tow)
    Probably better to use c=0

    But c=1 would also give a perpendicular vector - just easier when using c=0
    You cant use Z=0 this results in the vector 0,0,0.


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    (Original post by physicsmaths)
    You cant use Z=0 this results in the vector 0,0,0.


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    Oh yeah.

    I assumed it would be easier - haven't done this question since we went through the book back in Feb!

    But yeah you use c=1 sorry about that xD

    It can be anything BUT 0 and obviously 1 would be the easiest choice.
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    (Original post by Tow)
    A very poor hint - if it could be considered a hint at all.
    To be fair that can be used to form 3 simultaneous equations but that'll just be harder to solve

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    (Original post by Tow)
    Oh yeah.

    I assumed it would be easier - haven't done this question since we went through the book back in Feb!

    But yeah you use c=1 sorry about that xD

    It can be anything BUT 0 and obviously 1 would be the easiest choice.
    Correct.


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    useful material for the upcoming exams

    http://www.thestudentroom.co.uk/show....php?t=3353445

    http://www.thestudentroom.co.uk/show....php?t=3361867

    http://www.thestudentroom.co.uk/show....php?t=3361905
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    (Original post by Tow)
    Yeah definitely.

    But no question should require you to use the 'Loop' method.

    Integration by parts like this is fine:
    x^2sin(x)

    But integration by parts of this shouldn't come up:
    Sin(3x)cos(x)

    Unless you can rearrange to make it so that you can integrate it with using the loop method.
    I don't know the integration by loops method, what should I use to work this out?


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    (Original post by lam12)
    I don't know the integration by loops method, what should I use to work this out?


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    Work what out?
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    (Original post by Tow)
    Work what out?
    The integration of the sin2x whatever it was :-)


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    (Original post by lam12)
    The integration of the sin2x whatever it was :-)


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    I think a previous post said that you can rearrange to integrate so try that.

    I can't do it right now so if you still don't understand I'll do it when I'm free.
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    do we need to know the integration of tanx cotx secx and cosecs?
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    (Original post by crepole)
    do we need to know the integration of tanx cotx secx and cosecs?
    They are in the formula book. You could be asked to differentiate one of the ln ones to get to tanx or something along those lines.


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    Does anyone know why you need a minus sign infront of the constant when setting up a diff. equation and its decreasing? thanks
 
 
 
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