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    1- Find a vector v perpendicular to the vector u= 5i + 5j
    :|
    how do i do this...
    i know that
    v.u = |v||u|[email protected]
    when perpendicular [email protected] = cos(90) = 0
    so v.u=0
    so (5i + 5j) ( ai+ bj) = 0
    so 5a=0
    and 5b = 0
    wat do i do from here?
    any help??
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    You're not dotting them correctly. When you dot them, you get

    5a + 5b = 0
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    You've calculated your scalar product incorrectly.

    (5i+5j).(ai+bj) = 5a+5b.

    now since the magnitude of the vector is irrelevant, set a = 1 and work out b.
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    sweet! i did it

    how bout this?
    if u = ( 3, -4, -2) and v = ( -5, 3, -5) find the angle between them...
    so i do : u.v/ |u||v| [email protected]
    > u.v = -15-12+10 = -17
    > |u| = sqrt29 |v| = sqrt59
    so -17/(sqrt59 x sqrt29) = -0.410983
    so [email protected]= -0.410983
    hence @ = 114.2666
    except its wrong..
    does the angle have to be acute?
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    (Original post by valentine_strongrod)
    sweet! i did it

    how bout this?
    if u = ( 3, -4, -2) and v = ( -5, 3, -5) find the angle between them...
    so i do : u.v/ |u||v| [email protected]
    > u.v = -15-12+10 = -17
    > |u| = sqrt29 |v| = sqrt59
    so -17/(sqrt59 x sqrt29) = -0.410983
    so [email protected]= -0.410983
    hence @ = 114.2666
    except its wrong..
    does the angle have to be acute?
    Your answer is technically correct (there are two angles at an intersection of two lines), but the acute angle is the one required. Just do 180 minus your answer to get the acute angle.
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    still got it wrong...
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    what answer are you supposed to get?
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    What's our vector, Victor?
    • Thread Starter
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    heres the question
    fine the angle between u = ( 3, -4, -2) and v = ( -5, 3, -5)
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    a.b = ab cos x
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    (Original post by insparato)
    a.b = ab cos x
    i get the wrong answer
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    Post your workings then. I don't know what you've done wrong.
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    (Original post by insparato)
    Post your workings then. I don't know what you've done wrong.

    if u = ( 3, -4, -2) and v = ( -5, 3, -5) find the angle between them...
    so i do : u.v/ |u||v| [email protected]
    > u.v = -15-12+10 = -17
    > |u| = sqrt29 |v| = sqrt59
    so -17/(sqrt59 x sqrt29) = -0.410983
    so [email protected]= -0.410983
    hence @ = 114.2666
    except its wrong..
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    Whats the acute angle ? I think that's what its looking for.
    • Thread Starter
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    how do i find that?
    i did 180 - 114.266
    and still got it worng
 
 
 
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