Are congruent circles.
It is the radius. More than one are radii. All the way across is the diameter.
Since pies are circular, the pie shaped pieces will be more concentric circles.
Radius is the singular. The plural is Radii. 1 radius, more than 1 Radii.
Two or more angles that have the same measure are called congruent angles.
The trend for atomic radii shows that,moving across a period, the elements' atomic radii decrease making it more difficult to dislodge the outermost electron.
A polygon that has congruent sides and congruent angles is called a Regular polygon. If the number of sides is given, you can be more specific. Some examples: 3 congruent sides/angles = equilateral triangle 4 congruent sides/angles = square 5 congruent sides/angles = regular pentagon 6 congruent sides/angles = regular hexagon ...and so on, by adding "regular" in front of the shape's name.