Sine and Cosine are rebelling!

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Dubious Drewski

Could someone please tell me why the following code creates a square instead of a round ring?

Graphics 320, 200
For Local a = 0 To 900
Local x:Float = 20* Sin(Rnd(360))
Local y:Float = 20* Cos(Rnd(360))
Plot 160+x,100+y
Next
Flip
WaitKey

This works with any variable, but once I have an Rnd as
the angle, as above, it goes square, and I can't figure out why.

[edit]

For example, this code works perfectly fine:

Graphics 320, 200
For Local a = 0 To 360
Local x:Float = 20* Sin(a)
Local y:Float = 20* Cos(a)
Plot 160+x,100+y
Next
Flip
WaitKey