Lesson 8 of 15
A rectangle also has matching diagonals
Show on a big screenA carpenter checks a door frame with a tape, corner to corner. Why does that work?
Step 1 of 4
- A rectangle, 8 by 6. Both diagonals measure 10.
- Lean the top. The sides stay 8 and 6. The corners are no longer 90°.
- The diagonals are now about 12.2 and 7.2.
- Equal diagonals happen only at a right angle.
Equal readings show a rectangle, if both pairs of sides are parallel.
Rule: A parallelogram whose diagonals are equal is a rectangle. That is the carpenter's check. A door frame has parallel opposite sides. So it is a parallelogram. Equal tape readings then show all four corners are square.
Equal diagonals alone are not enough. A shape has a bottom of 8 and a height of 6. Its top is 2 long and sits in the middle. The top is 3 in from each end. Its diagonals are both about 7.8. Only one pair of its sides is parallel. So it is not a parallelogram.
Drag the slider to lean the frame. Find where the diagonals match.
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Try the lab to finish this lesson.