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Trigonometry Dot Product Question?

I am having a problem figuring out this dot product thing. i am to find the smallest angle between the pair of vectors <-3,2>,<4,5>. ok, so i have cosa=<-3,2><4,5> / |<-3,2>| |<4,5>|. i have a -2 for the top part of the fraction but i have no clue how to do the bottom. the answer says it's (sqrt13)(sqrt14) and then go from there. how do i get the square roots of 13 and 14?

5 Answers

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  • 1 decade ago
    Favorite Answer

    The norm of <x,y>, denoted |<x,y>|, is supposed to represent the length or strength of the vector <x,y>. Because of Pythagorean theorem reasons, this is defined to be sqrt(x^2 + y^2). Another way of expressing it is sqrt(length<x,y>|^2). Now imagine a right triangle with sides x and y. Then x^2 + y^2 = z^2, where z is the hypotenuse. The hypotenuse is the length we want, so we get sqrt(z^2) = sqrt(x^2 + y^2).

    For your problem. |(-3,2)| = sqrt(3^2+2^2) = sqrt(9+4) = sqrt(13). The other term is |<4,5>| = sqrt(4^2+5^2) = sqrt(41). You got the digits turned around the wrong way.

  • Rick
    Lv 5
    1 decade ago

    |<-3,2>| = sqrt(3*3 + 2*2) = sqrt(9 + 4) = sqrt(13)

    |<4,5>| = sqrt(4*4 + 5*5) = sqrt(16 + 25) = sqrt(41)

  • 1 decade ago

    The should be the norm value.

    The length of vector v is ||v||=sqrt(v1^2+v2^2+...vn^2).

    So ||<-3,2>||=sqrt((-3)^2+(2)^2) =sqrt(9+4)=sqrt(13)

    and

    ||<4,5>||=sqrt(4^2+5^2) =sqrt(16+25) =sqrt(41) .... you sure it is s'posed to be sqrt(14)?

  • Anonymous
    1 decade ago

    The bars on the bottom mean magnitude of a vector. The magnitude of vectro<x,y> = sqrt(x^2 + y^2)

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  • Anonymous
    4 years ago

    it relies upon, i wager, on the scope of your subject at this aspect. (a.b) supplies a huge style as its answer. a consistent huge style could have a dot product with c, if it is what your syllabus helps.

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