In MS-Word, you can type (r). It should automatically be converted to the "registered trademark" symbol, by Word's autocorrect feature. You can also use the "Insert Symbol" command - or, if you use Windows, open the CharMap application, find the symbol, then use copy & paste.
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A little R in a circle usually means "registered trademark".
An annulus. Area = pi (R2 - r2) when R is radius of larger circle and r is radius of smaller circle.
The area of a circle is the number of square units inside that circle, if each square in the circle to the left has an area of 1cm2, you could count the total number of squares to get the area of this circle. However, it is easier to use one the following formulas; A=.r²or A=pi times r times r, where A is the area and r is the radius.
You are describing a railroad crossing sign.
The director circle of a circle with radius r is a concentric circle with radius r*sqrt(2).