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I have to solve the following task: Write a Manipulate command containing the values ​​a, b, c and d of the matrix $\begin{pmatrix} a & b \\ c & d \end{pmatrix}$ each varied in the range [0,1]. Start with a and d at value 1 and b unch c at value 0. In the manipulate environment, plot the values ​​of all unit vectors after applying the matrix in a parametric plot. I wrote the following:

MatrixForm [M = {a, b}, {c, d}]
Manipulate [
ParametricPlot [
   M. {Cos [\ Phi], Sin [\ Phi]}, {\ Phi, 0, 2 Pi}, 
   PlotRange -> {{-1.4, 1.4}, {-1.4, 1.4}}], {{a, 1, "a"}, 0,
   1}, {{b, 0, "b"}, 0, 1}, {{c, 0, "c"}, 0, 1}, {{d, 1, "d"}, 0, 1} ]

But it does not work. Can you help me? Thank you

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1 Answer 1

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Maniputate wants to see its variables explicitly in the expressions. This should work:

M = {a, b, c, d} \[Function] {{a, b}, {c, d}};
Manipulate[
 ParametricPlot[
  M[a, b, c, d].{Cos[ϕ], Sin[ϕ]}, {ϕ, 0, 2 Pi}, 
  PlotRange -> {{-1.4, 1.4}, {-1.4, 1.4}}], {{a, 1, "a"}, 0, 
  1}, {{b, 0, "b"}, 0, 1}, {{c, 0, "c"}, 0, 1}, {{d, 1, "d"}, 0, 1}]
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  • $\begingroup$ It works like the Code I have wrote. I cannot see any graph. $\endgroup$
    – Kathe
    Commented Apr 11, 2019 at 13:24
  • $\begingroup$ How exactly does it look like? Maybe you have deactivated dynamic updating? Have a look in the "Evaluation" menue. $\endgroup$
    – Henrik Schumacher
    Commented Apr 11, 2019 at 13:43

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