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# rep for c4 answer. watch

1. (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
2. 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.
3. 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
4. p.s i hate it when people say rep available-- thats not why people help on this forum!!
5. when they are perpendicular, a.b = 0

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

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