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    (Original post by edexcel c4 book pg 79 q14)
    Two submarines are travelling in straight lines through the ocean. Relative to a fixed origin, the vector equetions of the two lines, l1 and l2, along which they travel are:

    r = 3i +4j -5k + ß(i -2j +2k)
    and r = 9i + j -2k + µ(4i +j -k)

    where ß and µ are scalars.

    Show that the submarines are moving in perpendicular directions.
    I know you have to use a.b = lal lbl costheta

    but for some reason it doesnt work out :confused:
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    For perpendicular lines you just show that the dot product of the direction vectors = 0.

    (1,-2 ,2).(4,1,-1) = 4 -2 -2 =0

    So yes they are perp.
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    when there perp a.b = 0 therefore (i -2j +2k).(4i +j -k) = 0
    ive just been doing these me self
    c4 is so fun, not
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    p.s i hate it when people say rep available-- thats not why people help on this forum!!
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    when they are perpendicular, a.b = 0

    multiply the coefficients and it should all equal zero.
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    oh i didnt u had to ignore the first bits. thx
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    oh i get it, coz those bits are the gradients
 
 
 
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