2d Shapes

Plane Geometry is the study of flat shapes that you see in pictures or on paper. These shapes include lines, circles, triangles, squares, and rectangles. They are called flat because they have only length and width.

A side is a straight line on a shape.

A vertex is a point where two sides meet.

A polygon is a flat shape that is closed, has only straight sides, and has sides that do not cross each other.

Look at the shapes below. Decide if each one is a polygon.\(\quad\)\(\quad\)\(\quad\)

A polygon is named by the number of sides it has. Here are some common polygons:

Triangle	Quadrilateral	Pentagon	Hexagon
3 sides	4 sides	5 sides	6 sides

A regular polygon has all sides the same length.\hspace{1cm}\hspace{1cm}

A rectangle is a quadrilateral with the opposite sides are equal in length.

A circle is every point on the edge as same distance from the center.
The radius is a line segment from the center to a point of the circle. The radius is also the length of this segment.
The diameter is a line segment that goes across the circle through the center, connecting two points on the circle. The diameter is also the length of this segment.

• The diameter is twice the radius: \(\textcolor{colorprop}{d}=2\times \textcolor{colordef}{r}\).
• The radius is half the diameter: \(\textcolor{colordef}{r}=\textcolor{colorprop}{d} \div 2\)