Posted by Anonymous on Sunday, March 30, 2008 at 9:27am.
For the first question I really need all help I can get. Thanks!
1. Given a and b are unit vectors,
a) if the angle between them is 60 degrees, calculate (6a+b) . (a2b)
b) if a+b = sqrt3, determine (2a5b) . (b+3a)
2. The vectors a = 3i  4j  k and b = 2i + 3j  6k are the diagonals of a parallelogram. Show that this parallelogram is a rhombus, and determine the lengths of the sides and angles between the sides.
3. If a and b are perpendicular, show that a^2 + b^2 = a + b^2. What is the usual name of this result.
b) If a and b are not perpendicular, and ab = c, express c^2 in terms of a and b. What is the usual name of this result?

Math please help  Reiny, Sunday, March 30, 2008 at 11:17am
You seem to post quite a few vector questions under the name of anonymous.
Are you the same person?
Please use a first name or some other nick to identify yourself.
Your first question uses the basic laws of vectors.
(2a5b)∙(b+3a) = 2a∙b + 6│a│^2  5│b│^2  15a∙b
= 6│a│^2  5│b│^2  13a∙b
we know │a│ and │b│ are 1 each and
a∙b = │a││b│cos60º
a∙b = 1*1*1/2
so 6│a│^2  5│b│^2  13a∙b
= 6*1  5*1  13*1/2
= 11/2
for 1. b) make a diagram and find cosß using sides 1,1,√3
that way you can find a∙b and follow my example of a)
2. In a parallelogram the diagonals bisect each other, but in a rhombus (which is a parallelogram) they bisect each other at right angles.
so take the dot product, and see if you get zero. (you will)
then 1/2 of vector a + 1/2 of vector b will give you a side
etc.
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