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# C4 Vectors help watch

1. Done part a)

not sure about b) and c)
i bet it's something really obvious but im just being very thick today!

thanks
2. I was terrible with vectors, but lemme try.

b) It's the angle between OD and OA/OB/OC as A, B and C are on the same line, so it doesn't matter.

So you use the dot product:

OD . OA/OB/OC = [OD][OA/OB/OC]cos(theta)

or you can do:

DO . AO/BO/CO = [DO][AO/BO/CO]cos(theta)

c) I'm more than hazy, so I can't remember, sorry. To prove something's perpendicular, the dot product needs to equal to 0.
3. b)
Angle = arccos (dotproduct of direction vectors of both / modulus of both direction vectors multiplied)

c) If OE is Perp to OD
then direction vectors of OE and OD gives a dot procut of zero...
this is coz if u see the above equation angle will be 90 (perpendicular) only when whats inside the arccos is zero (which happens when dot product is zero)...

Now try it,.

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Updated: February 8, 2010
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