Square corners: perpendicular lines
The edges of a page meet at a square corner. Can two lines cross so that all four angles match?
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The rule
Yes: lines that cross at a right angle make four equal angles.
Two lines that cross at a right angle are perpendicular. If one of the four angles is 90°, all four are. A small square marks the corner.
| If a = 90° | Why |
|---|---|
| b = 90° | a and b fill a straight line: 180° − 90° |
| c = 90° | c is across from a |
| d = 90° | d is across from b |
The edges at a window corner are perpendicular. So are the edges at a notebook corner.