Triangle: 3 sides, 3 angles
Quadrilateral: 4 sides, 4 angles
Pentagon: 5 sides
Hexagon: 6 sides
Heptagon: 7 sides
Octagon: 8 sides
Nonagon: 9 sides
Decagon: 10 sides
Hendecagon: 11 sides
Dodecagon: 12 sides
Circle: All points the same distance from the center; no sides or corners
Oval (ellipse): A stretched circle
Square: 4 equal sides and 4 right angles
Rectangle: 4 right angles; opposite sides equal
Rhombus: 4 equal sides; opposite angles equal
Parallelogram: Both pairs of opposite sides are parallel and equal
Trapezoid: A quadrilateral with at least one pair of parallel sides
Kite: A quadrilateral with two pairs of equal sides that meet side by side
Equilateral triangle: All 3 sides equal; all angles 60 degrees
Isosceles triangle: At least 2 equal sides and 2 equal angles
Scalene triangle: No equal sides
Right triangle: Has one 90-degree angle
Acute triangle: All three angles are less than 90 degrees
Obtuse triangle: Has one angle greater than 90 degrees
Regular polygon: A polygon with all sides equal and all angles equal
Irregular polygon: A polygon whose sides or angles are not all equal
Convex polygon: A polygon with no angle pointing inward
Concave polygon: A polygon with at least one angle pointing inward
Sum of angles in a triangle: 180 degrees
Sum of angles in a quadrilateral: 360 degrees
Sum of angles in a pentagon: 540 degrees
Sum of angles in a hexagon: 720 degrees
Sum of angles in an n-sided polygon: (n − 2) × 180 degrees
Each angle of a regular hexagon: 120 degrees
Each angle of a regular pentagon: 108 degrees
Each angle of a regular octagon: 135 degrees
Number of diagonals of a quadrilateral: 2
Number of diagonals of a pentagon: 5
Number of diagonals of a hexagon: 9
Diagonal: A segment joining two corners of a polygon that are not next to each other
Is a square a rectangle?: Yes. A square is a rectangle with all sides equal.
Is a square a rhombus?: Yes. A square is a rhombus with right angles.
Is a rectangle a parallelogram?: Yes. Its opposite sides are parallel.
Do the diagonals of a rectangle have equal lengths?: Yes
Do the diagonals of a rhombus meet at right angles?: Yes, and they bisect each other
Crescent: A moon shape formed by two arcs
Semicircle: Half of a circle
Quarter circle: A quarter of a circle; a 90-degree sector
Star shape (pentagram): A five-pointed star drawn with five straight lines

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