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In program Mathematica 11

x[t_] := Exp[-p*t]*Cos[c*t]
y[t_] := Exp[-q*t]*Sin[d*t]

ParametricPlot[{x[t], y[t]}, {t, 0, 2*Pi}]

But

enter image description here

What is going wrong?

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    $\begingroup$ YOu haven't specified values for p, c, q, or d. $\endgroup$ – march Nov 1 '16 at 20:47
  • $\begingroup$ For example {p,1,3} ? $\endgroup$ – Jonathan Nov 1 '16 at 21:25
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    $\begingroup$ I have no idea what you mean by {p, 1, 3}. The point that I was making is that you can't plot a function that has variables left undefined. If you put numbers in for p, c, q, and d, the plot will work just fine. $\endgroup$ – march Nov 1 '16 at 21:29
  • $\begingroup$ march means that you should assign values to p,c,q and d. Try for example {p,c,q,d}={1,2,3,4}. $\endgroup$ – anderstood Nov 1 '16 at 22:13
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You have to assign values to the parameters, e.g.

p = q = c = d = 1;

x[t_] := Exp[-p*t]*Cos[c*t]
y[t_] := Exp[-q*t]*Sin[d*t]

ParametricPlot[{x[t], y[t]}, {t, 0, 2*Pi}]

enter image description here

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