Hi,
this is very easy, if you know trigonometry.
But the most easiest way is to use functions from
"Numeric->Complex" palette.
Use first "Polar to complex" to convert polar coordinates to complex number, and then "Complex to Re/Im" to convert complex number to "X-Y" coordinates.
The example is attached.
Angle Theta in example must be in radians.
Good luck.
Oleg Chutko.