What is Square, rectangle, triangle?

Also known as: rectangle · square · triangle · circle · radius

Definition 18.3 Primary & Middle School Mathematics · Chapter 18 — Shapes and Right Angles

A rectangle has $4$ sides and $4$ right angles; its opposite sides have the same length.
A square is a rectangle whose $4$ sides all have the same length.
A triangle has $3$ sides; a right triangle has one right angle.
A circle is drawn with the compass: all its points are at the same distance (the radius) from the center.

The shape gallery, with their marks: little squares for right angles, small ticks for equal sides.

Examples

Example 18.4 (Why marks matter)

A shape drawn slightly tilted can look like a square without being one. The marks tell the truth: $4$ right-angle squares $+$ $4$ tick marks $=$ a real square. When you draw, put the marks; when you read a figure, trust only the marks and the given measures.

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Examples

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