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October 10, 2015
Posted by **angles** on Wednesday, August 24, 2011 at 10:45am.

I'm not sure what to do here; I believe that I use to definition of a dot product (or maybe cross product) with relation to the trig function (cos for dot, sin for cross) but for whatever reason I can't get my answer to come out.

- calculus -
**Reiny**, Wednesday, August 24, 2011 at 1:01pmlet such a vector be (a,b)

then (a,b)⋅(3,4) = |(a,b)||(3,4)|cos60°

3a + 4b = 5√(a^2 + b^2)(1/2)

6a+8b = 5√(a^2+b^2)

let a= 1 , any non-zero number will do

6+8b = 5√(1+b^2)

square both sides

36+96b +64 b^2 = 25 + 25b^2

39b^2 + 96b + 11 = 0

b = (-96 ± √7500)/48

= appr. -2.341 or -0.1205

so two vectors could be (1, -2.341) or (1, -.1205)

but you want a unit vector, so using the second point,

|(1,-.1205)| = 1.00723

giving us a unit vector of (1/1.00723 , -.1205/1.00723)

or appr. (.9928 , -.1196)

check:

(3,4).(.9928,-.1196) = 2.5

|(3,4)||(.9928,-.1196)cos60 = 2.5

reverse the direction of the above vector to get

(-.9928, .1196) for a second vector, or

let a = some other value and repeat the above steps