Vector linear combination problem












2












$begingroup$


Diagram




Point $A$ and $B$ have position vectors $vec a$ and $vec b$ respectively relative to an orgin $O$.



The point $D$ is such that $overrightarrow{OD} = koverrightarrow{OA}$ and the point $E$ is such that $overrightarrow{AE} = loverrightarrow{AB}$.



The line segments $BD$ and $OE$ intersect at $X$.



If $overrightarrow{OX} = frac{2}{5}overrightarrow{OE}$ and $overrightarrow{XB} = dfrac{4}{5}overrightarrow{DB}$.



Express $overrightarrow{OX}$ and $XB$ in terms of $vec a, vec b, k, l$ and hence evaluate $k$ and $l$.




I have worked out most of the problem but can't figure out how to evaluate $l$.



Using ratio theorem, I got $overrightarrow{OX}$ as,



$$
overrightarrow{OX} = dfrac{2}{5}Big[(1-l)a + lbBig]
$$



And similarly, $overrightarrow{XB}$



$$
overrightarrow{XB} = dfrac{4}{5}(b - ka)
$$



Then using,
$$
begin{align}
overrightarrow{OX} + overrightarrow{XB} &= overrightarrow{OB} \
dfrac{2}{5}Big[(1-l)a + lbBig] + dfrac{4}{5}(b - ka) &= b\
\
text{...}\
\
[2(1-l) - 4k]a &= (1-2l)b\
2(1-l)- 4k &= 1 - 2l & text{(a and b are non-zero and non-parallel)}\
-4k &= -1 \
k &= dfrac{1}{4}\
end{align}
$$



I can't seem to figure out how to get $l$. I tried $overrightarrow{DA} + overrightarrow{AE} + overrightarrow{E} = overrightarrow{DX}$ and $overrightarrow{OA} + overrightarrow{AE} = overrightarrow{OE}$, these just give an equality statement.




How do I evaluate $l$?




Thanks for your help.










share|cite|improve this question











$endgroup$

















    2












    $begingroup$


    Diagram




    Point $A$ and $B$ have position vectors $vec a$ and $vec b$ respectively relative to an orgin $O$.



    The point $D$ is such that $overrightarrow{OD} = koverrightarrow{OA}$ and the point $E$ is such that $overrightarrow{AE} = loverrightarrow{AB}$.



    The line segments $BD$ and $OE$ intersect at $X$.



    If $overrightarrow{OX} = frac{2}{5}overrightarrow{OE}$ and $overrightarrow{XB} = dfrac{4}{5}overrightarrow{DB}$.



    Express $overrightarrow{OX}$ and $XB$ in terms of $vec a, vec b, k, l$ and hence evaluate $k$ and $l$.




    I have worked out most of the problem but can't figure out how to evaluate $l$.



    Using ratio theorem, I got $overrightarrow{OX}$ as,



    $$
    overrightarrow{OX} = dfrac{2}{5}Big[(1-l)a + lbBig]
    $$



    And similarly, $overrightarrow{XB}$



    $$
    overrightarrow{XB} = dfrac{4}{5}(b - ka)
    $$



    Then using,
    $$
    begin{align}
    overrightarrow{OX} + overrightarrow{XB} &= overrightarrow{OB} \
    dfrac{2}{5}Big[(1-l)a + lbBig] + dfrac{4}{5}(b - ka) &= b\
    \
    text{...}\
    \
    [2(1-l) - 4k]a &= (1-2l)b\
    2(1-l)- 4k &= 1 - 2l & text{(a and b are non-zero and non-parallel)}\
    -4k &= -1 \
    k &= dfrac{1}{4}\
    end{align}
    $$



    I can't seem to figure out how to get $l$. I tried $overrightarrow{DA} + overrightarrow{AE} + overrightarrow{E} = overrightarrow{DX}$ and $overrightarrow{OA} + overrightarrow{AE} = overrightarrow{OE}$, these just give an equality statement.




    How do I evaluate $l$?




    Thanks for your help.










    share|cite|improve this question











    $endgroup$















      2












      2








      2





      $begingroup$


      Diagram




      Point $A$ and $B$ have position vectors $vec a$ and $vec b$ respectively relative to an orgin $O$.



      The point $D$ is such that $overrightarrow{OD} = koverrightarrow{OA}$ and the point $E$ is such that $overrightarrow{AE} = loverrightarrow{AB}$.



      The line segments $BD$ and $OE$ intersect at $X$.



      If $overrightarrow{OX} = frac{2}{5}overrightarrow{OE}$ and $overrightarrow{XB} = dfrac{4}{5}overrightarrow{DB}$.



      Express $overrightarrow{OX}$ and $XB$ in terms of $vec a, vec b, k, l$ and hence evaluate $k$ and $l$.




      I have worked out most of the problem but can't figure out how to evaluate $l$.



      Using ratio theorem, I got $overrightarrow{OX}$ as,



      $$
      overrightarrow{OX} = dfrac{2}{5}Big[(1-l)a + lbBig]
      $$



      And similarly, $overrightarrow{XB}$



      $$
      overrightarrow{XB} = dfrac{4}{5}(b - ka)
      $$



      Then using,
      $$
      begin{align}
      overrightarrow{OX} + overrightarrow{XB} &= overrightarrow{OB} \
      dfrac{2}{5}Big[(1-l)a + lbBig] + dfrac{4}{5}(b - ka) &= b\
      \
      text{...}\
      \
      [2(1-l) - 4k]a &= (1-2l)b\
      2(1-l)- 4k &= 1 - 2l & text{(a and b are non-zero and non-parallel)}\
      -4k &= -1 \
      k &= dfrac{1}{4}\
      end{align}
      $$



      I can't seem to figure out how to get $l$. I tried $overrightarrow{DA} + overrightarrow{AE} + overrightarrow{E} = overrightarrow{DX}$ and $overrightarrow{OA} + overrightarrow{AE} = overrightarrow{OE}$, these just give an equality statement.




      How do I evaluate $l$?




      Thanks for your help.










      share|cite|improve this question











      $endgroup$




      Diagram




      Point $A$ and $B$ have position vectors $vec a$ and $vec b$ respectively relative to an orgin $O$.



      The point $D$ is such that $overrightarrow{OD} = koverrightarrow{OA}$ and the point $E$ is such that $overrightarrow{AE} = loverrightarrow{AB}$.



      The line segments $BD$ and $OE$ intersect at $X$.



      If $overrightarrow{OX} = frac{2}{5}overrightarrow{OE}$ and $overrightarrow{XB} = dfrac{4}{5}overrightarrow{DB}$.



      Express $overrightarrow{OX}$ and $XB$ in terms of $vec a, vec b, k, l$ and hence evaluate $k$ and $l$.




      I have worked out most of the problem but can't figure out how to evaluate $l$.



      Using ratio theorem, I got $overrightarrow{OX}$ as,



      $$
      overrightarrow{OX} = dfrac{2}{5}Big[(1-l)a + lbBig]
      $$



      And similarly, $overrightarrow{XB}$



      $$
      overrightarrow{XB} = dfrac{4}{5}(b - ka)
      $$



      Then using,
      $$
      begin{align}
      overrightarrow{OX} + overrightarrow{XB} &= overrightarrow{OB} \
      dfrac{2}{5}Big[(1-l)a + lbBig] + dfrac{4}{5}(b - ka) &= b\
      \
      text{...}\
      \
      [2(1-l) - 4k]a &= (1-2l)b\
      2(1-l)- 4k &= 1 - 2l & text{(a and b are non-zero and non-parallel)}\
      -4k &= -1 \
      k &= dfrac{1}{4}\
      end{align}
      $$



      I can't seem to figure out how to get $l$. I tried $overrightarrow{DA} + overrightarrow{AE} + overrightarrow{E} = overrightarrow{DX}$ and $overrightarrow{OA} + overrightarrow{AE} = overrightarrow{OE}$, these just give an equality statement.




      How do I evaluate $l$?




      Thanks for your help.







      algebra-precalculus






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      share|cite|improve this question













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      share|cite|improve this question








      edited Dec 11 '18 at 22:24









      Glorfindel

      3,41981830




      3,41981830










      asked Jul 20 '11 at 14:37









      mathguy80mathguy80

      621720




      621720






















          1 Answer
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          $begingroup$

          At some point you said
          $$
          2(1-ell)- 4k = 1-2 ell
          $$
          But you should've noticed that you could say that because
          $$
          2(1-ell) - 4k = 0 = 1-2 ell.
          $$
          I think that gives you $ell = 1/2$.



          Hope that helps,






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Thanks. Can you please clarify why you equated it to 0?
            $endgroup$
            – mathguy80
            Jul 20 '11 at 15:05










          • $begingroup$
            Well, since you have a multiple of $a$ and a multiple of $b$ that are equal, and that those two vectors are not colinear, it must be because they are both the zero vectors, hence their coefficients are both zero. Isn't that why you said that $2(1-ell) - 4k = 1-2ell$ in the first place?
            $endgroup$
            – Patrick Da Silva
            Jul 20 '11 at 15:19










          • $begingroup$
            The proof that I was taught is, $$ $$ Let $a$ and $b$ be two non-zero and non-parallel vectors. If $$alpha a + beta b = lambda a + mu b$$ for some $alpha, beta, lambda, mu in mathbb{R}$, then $$(alpha - lambda)a = (mu - beta)b$$ and hence, $$alpha = lambda text{ and } mu = beta$$. It was assumed that a and b are not zero vectors. Does that hold for zero vectors also?
            $endgroup$
            – mathguy80
            Jul 20 '11 at 16:06










          • $begingroup$
            It does not, indeed. Having $(1-2) vec{0} = vec 0$ does not imply $1=2$. =) But the idea in your proof was that since $a$ and $b$ cannot be parallel, it is impossible for them that a multiple of $a$ is equal to a multiple of $b$, unless they are both $0$, which leads to both coefficients being $0$.
            $endgroup$
            – Patrick Da Silva
            Jul 20 '11 at 16:15










          • $begingroup$
            Ah! Thanks, that makes sense. I was beginning to think I had learned some basic ideas incorrectly!
            $endgroup$
            – mathguy80
            Jul 21 '11 at 2:51











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          1 Answer
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          1 Answer
          1






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          active

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          1












          $begingroup$

          At some point you said
          $$
          2(1-ell)- 4k = 1-2 ell
          $$
          But you should've noticed that you could say that because
          $$
          2(1-ell) - 4k = 0 = 1-2 ell.
          $$
          I think that gives you $ell = 1/2$.



          Hope that helps,






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Thanks. Can you please clarify why you equated it to 0?
            $endgroup$
            – mathguy80
            Jul 20 '11 at 15:05










          • $begingroup$
            Well, since you have a multiple of $a$ and a multiple of $b$ that are equal, and that those two vectors are not colinear, it must be because they are both the zero vectors, hence their coefficients are both zero. Isn't that why you said that $2(1-ell) - 4k = 1-2ell$ in the first place?
            $endgroup$
            – Patrick Da Silva
            Jul 20 '11 at 15:19










          • $begingroup$
            The proof that I was taught is, $$ $$ Let $a$ and $b$ be two non-zero and non-parallel vectors. If $$alpha a + beta b = lambda a + mu b$$ for some $alpha, beta, lambda, mu in mathbb{R}$, then $$(alpha - lambda)a = (mu - beta)b$$ and hence, $$alpha = lambda text{ and } mu = beta$$. It was assumed that a and b are not zero vectors. Does that hold for zero vectors also?
            $endgroup$
            – mathguy80
            Jul 20 '11 at 16:06










          • $begingroup$
            It does not, indeed. Having $(1-2) vec{0} = vec 0$ does not imply $1=2$. =) But the idea in your proof was that since $a$ and $b$ cannot be parallel, it is impossible for them that a multiple of $a$ is equal to a multiple of $b$, unless they are both $0$, which leads to both coefficients being $0$.
            $endgroup$
            – Patrick Da Silva
            Jul 20 '11 at 16:15










          • $begingroup$
            Ah! Thanks, that makes sense. I was beginning to think I had learned some basic ideas incorrectly!
            $endgroup$
            – mathguy80
            Jul 21 '11 at 2:51
















          1












          $begingroup$

          At some point you said
          $$
          2(1-ell)- 4k = 1-2 ell
          $$
          But you should've noticed that you could say that because
          $$
          2(1-ell) - 4k = 0 = 1-2 ell.
          $$
          I think that gives you $ell = 1/2$.



          Hope that helps,






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Thanks. Can you please clarify why you equated it to 0?
            $endgroup$
            – mathguy80
            Jul 20 '11 at 15:05










          • $begingroup$
            Well, since you have a multiple of $a$ and a multiple of $b$ that are equal, and that those two vectors are not colinear, it must be because they are both the zero vectors, hence their coefficients are both zero. Isn't that why you said that $2(1-ell) - 4k = 1-2ell$ in the first place?
            $endgroup$
            – Patrick Da Silva
            Jul 20 '11 at 15:19










          • $begingroup$
            The proof that I was taught is, $$ $$ Let $a$ and $b$ be two non-zero and non-parallel vectors. If $$alpha a + beta b = lambda a + mu b$$ for some $alpha, beta, lambda, mu in mathbb{R}$, then $$(alpha - lambda)a = (mu - beta)b$$ and hence, $$alpha = lambda text{ and } mu = beta$$. It was assumed that a and b are not zero vectors. Does that hold for zero vectors also?
            $endgroup$
            – mathguy80
            Jul 20 '11 at 16:06










          • $begingroup$
            It does not, indeed. Having $(1-2) vec{0} = vec 0$ does not imply $1=2$. =) But the idea in your proof was that since $a$ and $b$ cannot be parallel, it is impossible for them that a multiple of $a$ is equal to a multiple of $b$, unless they are both $0$, which leads to both coefficients being $0$.
            $endgroup$
            – Patrick Da Silva
            Jul 20 '11 at 16:15










          • $begingroup$
            Ah! Thanks, that makes sense. I was beginning to think I had learned some basic ideas incorrectly!
            $endgroup$
            – mathguy80
            Jul 21 '11 at 2:51














          1












          1








          1





          $begingroup$

          At some point you said
          $$
          2(1-ell)- 4k = 1-2 ell
          $$
          But you should've noticed that you could say that because
          $$
          2(1-ell) - 4k = 0 = 1-2 ell.
          $$
          I think that gives you $ell = 1/2$.



          Hope that helps,






          share|cite|improve this answer









          $endgroup$



          At some point you said
          $$
          2(1-ell)- 4k = 1-2 ell
          $$
          But you should've noticed that you could say that because
          $$
          2(1-ell) - 4k = 0 = 1-2 ell.
          $$
          I think that gives you $ell = 1/2$.



          Hope that helps,







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Jul 20 '11 at 14:50









          Patrick Da SilvaPatrick Da Silva

          32k354106




          32k354106












          • $begingroup$
            Thanks. Can you please clarify why you equated it to 0?
            $endgroup$
            – mathguy80
            Jul 20 '11 at 15:05










          • $begingroup$
            Well, since you have a multiple of $a$ and a multiple of $b$ that are equal, and that those two vectors are not colinear, it must be because they are both the zero vectors, hence their coefficients are both zero. Isn't that why you said that $2(1-ell) - 4k = 1-2ell$ in the first place?
            $endgroup$
            – Patrick Da Silva
            Jul 20 '11 at 15:19










          • $begingroup$
            The proof that I was taught is, $$ $$ Let $a$ and $b$ be two non-zero and non-parallel vectors. If $$alpha a + beta b = lambda a + mu b$$ for some $alpha, beta, lambda, mu in mathbb{R}$, then $$(alpha - lambda)a = (mu - beta)b$$ and hence, $$alpha = lambda text{ and } mu = beta$$. It was assumed that a and b are not zero vectors. Does that hold for zero vectors also?
            $endgroup$
            – mathguy80
            Jul 20 '11 at 16:06










          • $begingroup$
            It does not, indeed. Having $(1-2) vec{0} = vec 0$ does not imply $1=2$. =) But the idea in your proof was that since $a$ and $b$ cannot be parallel, it is impossible for them that a multiple of $a$ is equal to a multiple of $b$, unless they are both $0$, which leads to both coefficients being $0$.
            $endgroup$
            – Patrick Da Silva
            Jul 20 '11 at 16:15










          • $begingroup$
            Ah! Thanks, that makes sense. I was beginning to think I had learned some basic ideas incorrectly!
            $endgroup$
            – mathguy80
            Jul 21 '11 at 2:51


















          • $begingroup$
            Thanks. Can you please clarify why you equated it to 0?
            $endgroup$
            – mathguy80
            Jul 20 '11 at 15:05










          • $begingroup$
            Well, since you have a multiple of $a$ and a multiple of $b$ that are equal, and that those two vectors are not colinear, it must be because they are both the zero vectors, hence their coefficients are both zero. Isn't that why you said that $2(1-ell) - 4k = 1-2ell$ in the first place?
            $endgroup$
            – Patrick Da Silva
            Jul 20 '11 at 15:19










          • $begingroup$
            The proof that I was taught is, $$ $$ Let $a$ and $b$ be two non-zero and non-parallel vectors. If $$alpha a + beta b = lambda a + mu b$$ for some $alpha, beta, lambda, mu in mathbb{R}$, then $$(alpha - lambda)a = (mu - beta)b$$ and hence, $$alpha = lambda text{ and } mu = beta$$. It was assumed that a and b are not zero vectors. Does that hold for zero vectors also?
            $endgroup$
            – mathguy80
            Jul 20 '11 at 16:06










          • $begingroup$
            It does not, indeed. Having $(1-2) vec{0} = vec 0$ does not imply $1=2$. =) But the idea in your proof was that since $a$ and $b$ cannot be parallel, it is impossible for them that a multiple of $a$ is equal to a multiple of $b$, unless they are both $0$, which leads to both coefficients being $0$.
            $endgroup$
            – Patrick Da Silva
            Jul 20 '11 at 16:15










          • $begingroup$
            Ah! Thanks, that makes sense. I was beginning to think I had learned some basic ideas incorrectly!
            $endgroup$
            – mathguy80
            Jul 21 '11 at 2:51
















          $begingroup$
          Thanks. Can you please clarify why you equated it to 0?
          $endgroup$
          – mathguy80
          Jul 20 '11 at 15:05




          $begingroup$
          Thanks. Can you please clarify why you equated it to 0?
          $endgroup$
          – mathguy80
          Jul 20 '11 at 15:05












          $begingroup$
          Well, since you have a multiple of $a$ and a multiple of $b$ that are equal, and that those two vectors are not colinear, it must be because they are both the zero vectors, hence their coefficients are both zero. Isn't that why you said that $2(1-ell) - 4k = 1-2ell$ in the first place?
          $endgroup$
          – Patrick Da Silva
          Jul 20 '11 at 15:19




          $begingroup$
          Well, since you have a multiple of $a$ and a multiple of $b$ that are equal, and that those two vectors are not colinear, it must be because they are both the zero vectors, hence their coefficients are both zero. Isn't that why you said that $2(1-ell) - 4k = 1-2ell$ in the first place?
          $endgroup$
          – Patrick Da Silva
          Jul 20 '11 at 15:19












          $begingroup$
          The proof that I was taught is, $$ $$ Let $a$ and $b$ be two non-zero and non-parallel vectors. If $$alpha a + beta b = lambda a + mu b$$ for some $alpha, beta, lambda, mu in mathbb{R}$, then $$(alpha - lambda)a = (mu - beta)b$$ and hence, $$alpha = lambda text{ and } mu = beta$$. It was assumed that a and b are not zero vectors. Does that hold for zero vectors also?
          $endgroup$
          – mathguy80
          Jul 20 '11 at 16:06




          $begingroup$
          The proof that I was taught is, $$ $$ Let $a$ and $b$ be two non-zero and non-parallel vectors. If $$alpha a + beta b = lambda a + mu b$$ for some $alpha, beta, lambda, mu in mathbb{R}$, then $$(alpha - lambda)a = (mu - beta)b$$ and hence, $$alpha = lambda text{ and } mu = beta$$. It was assumed that a and b are not zero vectors. Does that hold for zero vectors also?
          $endgroup$
          – mathguy80
          Jul 20 '11 at 16:06












          $begingroup$
          It does not, indeed. Having $(1-2) vec{0} = vec 0$ does not imply $1=2$. =) But the idea in your proof was that since $a$ and $b$ cannot be parallel, it is impossible for them that a multiple of $a$ is equal to a multiple of $b$, unless they are both $0$, which leads to both coefficients being $0$.
          $endgroup$
          – Patrick Da Silva
          Jul 20 '11 at 16:15




          $begingroup$
          It does not, indeed. Having $(1-2) vec{0} = vec 0$ does not imply $1=2$. =) But the idea in your proof was that since $a$ and $b$ cannot be parallel, it is impossible for them that a multiple of $a$ is equal to a multiple of $b$, unless they are both $0$, which leads to both coefficients being $0$.
          $endgroup$
          – Patrick Da Silva
          Jul 20 '11 at 16:15












          $begingroup$
          Ah! Thanks, that makes sense. I was beginning to think I had learned some basic ideas incorrectly!
          $endgroup$
          – mathguy80
          Jul 21 '11 at 2:51




          $begingroup$
          Ah! Thanks, that makes sense. I was beginning to think I had learned some basic ideas incorrectly!
          $endgroup$
          – mathguy80
          Jul 21 '11 at 2:51


















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