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heureka

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 #2
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the coordinates of the points A and B are (-4,5) and (-5,-4) respectively.A' is reflection image of A with respect to y-axis.B is rotated anticlockwise about the origin O through 90     to B'

isA'B perpendicular to AB'?explain your answer.

\small{\text{  The vector dot product $\vec{A'B} *\vec{AB'}=0$, if $\vec{A'B}$ perpendicular $\vec{AB'}$ .  }}\\$   \small{\text{  We calculate: $\vec{A'B} = \vec{A'}-\vec{B}=\left(\begin{array}{c}4\\5\end{array}\right)-\left(\begin{array}{c}-5\\-4\end{array}\right)=\left(\begin{array}{c}4-(-5)\\5-(-4)\end{array}\right)}=\left(\begin{array}{c}9\\9\end{array}\right)$  }}$\\$  \small{\text{  and calculate: $\vec{AB'} = \vec{A}-\vec{B'}=\left(\begin{array}{c}-4\\5\end{array}\right)-\left(\begin{array}{c}4\\-5\end{array}\right)=\left(\begin{array}{c}-4-4)\\5-(-5)\end{array}\right)}=  \left(\begin{array}{c}-8\\10\end{array}\right)  $  }}$\\$  \small{\text{  $\vec{A'B} *\vec{AB'} =\left(\begin{array}{c}9\\9\end{array}\right)  *\left(\begin{array}{c}-8\\10\end{array}\right) =9*(-8)+9*10=-72+90=18  $  }}$\\$  \small{\text{  $18 \ne 0 $ so $ \vec{A'B} $ not perpendicular to $ \vec{AB'}$  }}

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15.01.2015