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Find all vectors in R^3 that are perpendicular to [1; 3; 1] 
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#1
Sep1206, 01:07 AM

P: 492

THE PROBLEM:
The dot product is: [tex]\overrightarrow{x}\,=\,\left[ \begin{array}{c} x_1 \\ x_2 \\ \vdots \\ x_n \end{array} \right][/tex] [tex]\overrightarrow{y}\,=\,\left[ \begin{array}{c} y_1 \\ y_2 \\ \vdots \\ y_n \end{array} \right][/tex] in [tex]\mathbb{R}^n[/tex]: [tex]\overrightarrow{x}\,\cdot\,\overrightarrow{y}\,=\,x_1\,y_1\,+\,x_2\,y_2 \,+\,\ldots\,+\,x_n\,y_n[/tex] If the scalar [itex]\overrightarrow{x}\,\cdot\,\overrightarrow{y}[/itex] is equal to zero, the vectors are perpendicular. Find all vectors in [itex]\mathbb{R}^3[/itex] that are perpendicular to [tex]\left[ \begin{array}{c} 1 \\ 3 \\ 1 \end{array} \right][/tex]. Draw a sketch as well. MY WORK SO FAR: [tex]\left[ \begin{array}{c} 1 \\ 3 \\ 1 \end{array} \right]\,\cdot\,\left[ \begin{array}{c} x \\ y \\ z \end{array} \right]\,=\,0[/tex] [tex]x\,+\,3\,y\,\,z\,=\,0[/tex] [tex]z\,=\,x\,+\,3\,y[/tex] Let s = x and t = y [tex]z\,=\,s\,+\,3\,t[/tex] [tex]\left[ \begin{array}{c} 1 \\ 3 \\ 1 \end{array} \right]\,\cdot\,\left[ \begin{array}{c} s \\ t \\ s\,+\,3\,t \end{array} \right]\,=\,0[/tex] Does the above look right? 


#2
Sep1206, 09:23 AM

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P: 39,503

Yes, that looks good but where is your answer? Since the problem asks for the set of all vectors perpendicular to [1, 3, 1] your answer should be something like "All vectors satisfying" or "all vectors spanned by". You have already calculated that a vector [x, y, z] in that space must satisfy x+ 3y z= 0; just say that. You have also calculated form that that z= x+ 3y and got [s, t, s+3t] as a "representative" vector. You could answer "all vectors of the form [s, t, s+ 3t] where s and t can be any real numbers. Finally, since [s, t, s+ 3t]= s[1, 0, 1]+ t[0, 1, 3], you could answer "the subspace spanned by [1, 0, 1] and [0, 1, 3]."
Your picture, of course, would be the plane x+ 3y z= 0. 


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