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Math Help - [SOLVED] vector Problem

  1. #1
    a.a
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    Unhappy [SOLVED] vector Problem

    Given ^a and ^b are unit vectors and magnitude of ^a + ^ b is sqrt. 3 then determine (2^a - 5^b) dot (^b + 3^a)

    From previous parts we are given that the angle os 60 degrees.

    So far i.ve got:

    (2^a - 5^b) dot (^b + 3^a) = 6 times the magnitude of ^a squred - 5 times the magnitude of ^b squared - 13 ^a dot ^ b

    Im not sure where to go from here....
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  2. #2
    Moo
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    Hello,

    Quote Originally Posted by a.a View Post
    Given ^a and ^b are unit vectors and magnitude of ^a + ^ b is sqrt. 3 then determine (2^a - 5^b) dot (^b + 3^a)

    From previous parts we are given that the angle os 60 degrees.

    So far i.ve got:

    (2^a - 5^b) dot (^b + 3^a) = 6 times the magnitude of ^a squred - 5 times the magnitude of ^b squared - 13 ^a dot ^ b

    Im not sure where to go from here....
    Since the angle is 60 (which is in harmony with the magnitude of \vec{a}+\vec{b}, \cos \theta=\frac 12.

    Therefore, \vec{a}.\vec{b}=|\vec{a}| |\vec{b}| \cos \theta=\frac 12 |\vec{a}| |\vec{b}|

    Since \vec{a} and \vec{b} are unit vectors, their magnitude is 1 !

    Can you conclude ?
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  3. #3
    a.a
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    ans

    I got -11/2 as the answer....?
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  4. #4
    Moo
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    Quote Originally Posted by a.a View Post
    I got -11/2 as the answer....?
    I get it too ^^
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