Identify and define perpendicular lines as a special case of intersection where all four angles are 90°.
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Introduction to perpendicular lines.
Imagine two intersecting lines that are perfectly balanced, creating four identical angles around their intersection point.
Since a full circle around any intersection is , what is the measure of each individual angle? If we divide by 4, we find that every angle must be exactly !
Needs Fig 5.4 to show the 90-degree square notation, and Fig 5.5 to show segments.
Lines are perpendicular when they form a 90° angle, marked by a square. They don't have to be perfectly vertical and horizontal to be perpendicular!
Line segments intersecting vs lines.
Unlike infinite lines, line segments have clear start and end boundaries. When we look at shapes or everyday objects, we often see segments meeting at a specific endpoint.
For example, imagine two segments, FG and FH. They meet precisely at the shared endpoint F, forming a specific angle—for instance, .
Identify perpendicular lines on a dot grid.
Activity 1 asks us to identify perpendicular lines drawn on a dot grid. Think about the properties of the grid itself to help you find the angles.
State the visual cue you would look for.
Why does the structure of the dot grid guarantee this angle?
Where else might you use a grid system to ensure perfect right angles?