# How can I get dotted boundaries on a RegionPlot?

How do I plot something like $$\{(x,y): 1 \leq x^2 + y^2 < 9\}?$$

I want to be able to make the boundary $x^2 + y^2 = 9$ dotted line and the unit circle filled and coloured region in between? I tried using

RegionPlot[1 <= x^2 + y^2 < 9, {x, -3.5, 3.5}, {y, -3.5, 3.5},
Frame -> False, AxesOrigin -> {0, 0}, Axes -> True]


But that gave me a bolded boundary on $x^2 + y^2 = 9$

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All you're missing is the option for BoundaryStyle

RegionPlot[1 <= x^2 + y^2 < 9, {x, -3.5, 3.5}, {y, -3.5, 3.5},
Frame -> False, AxesOrigin -> {0, 0}, Axes -> True,
BoundaryStyle -> {Dotted, Thickness[0.005]}]


If you want the unit circle filled, you can RegionPlot that separately:

Show[RegionPlot[1 <= x^2 + y^2 < 9, {x, -3.5, 3.5}, {y, -3.5, 3.5},
Frame -> False, AxesOrigin -> {0, 0}, Axes -> True,
BoundaryStyle -> {Dotted, Thickness[0.005]}],
RegionPlot[x^2 + y^2 < 1, {x, -3.5, 3.5}, {y, -3.5, 3.5},
PlotStyle -> Red, BoundaryStyle -> {Dotted, Thickness[0.005]}]]


You can find all these (and more) in the help for RegionPlot.

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Your second code gives me a red filled unit circle. – Hawk Jan 24 '14 at 2:40
Nvm I got it fixed. thanks – Hawk Jan 24 '14 at 2:47
You did say "and the unit circle filled" so I filled the unit circle with Red... or not... – bill s Jan 24 '14 at 3:26
No that would be the unit disk. – Hawk Jan 25 '14 at 2:22