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**Rotating a figure around its center**

Hi

I have this here this here linear transformation:

[tex](x,y) \rightarrow \left[ \begin{array}{cc} -cos( 2 \theta ) & sin(2 \theta) \\ sin(2 \theta) & cos(2 \theta) \end{array} \right] \left[ \begin{array}{c} x \\ y \end{array} \right][/tex]

Turns the figure recthangel A(3,1) B(5/2, 2) , (9/2, 3) , D(5,2)

90 degrees clockwise from the figures current position.

My questions is how do I change the transformation so it turns the figure around the figures center ???

Do I add the figures center like this

[tex](x,y) \rightarrow \left[ \begin{array}{cc} -cos( 2 \theta ) & sin(2 \theta) \\ sin(2 \theta) & cos(2 \theta) \end{array} \right] \left[ \begin{array}{c} x \\ y \end{array} \right] + \left[ \begin{array}{c} a \\ b \end{array} \right][/tex]

Fred

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