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Quadrilaterals

Diagonals that cut each other in half

Draw a parallelogram. Join each corner to the corner opposite it. These two lines are the diagonals. Where do they cross?

A parallelogram, both diagonals drawn, crossing at O.
Step 1 of 4
  1. A parallelogram, both diagonals drawn, crossing at O.
  2. One diagonal splits into two pieces: 4 and 4.
  3. The other splits into two pieces: 3 and 3.
  4. Each diagonal is cut exactly in half.

They cross at the middle of each diagonal. A parallelogram's diagonals halve each other.

Rule: A parallelogram's two diagonals bisect each other. Each diagonal is cut into two equal halves at the crossing point.

Halving does not make the two diagonals equal. One diagonal is 4 + 4 = 8 long. The other is 3 + 3 = 6 long. Halving and equal length are two separate facts.

A shape with exactly one parallel pair does not halve its diagonals.

Drag a corner. Watch the pieces of each diagonal. Switch the toggle off, and one corner moves alone.

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This track

  1. 0 Start here
  2. 1 Four straight sides, joined up
  3. 2 The four angles come to 360°
  4. 3 A line to the corner across
  5. 4 Five names and what each one demands
  6. 5 What a parallelogram gives you
  7. 6 Checkpoint: sides, angles and the family
  8. 7 Diagonals that cut each other in half
  9. 8 A rectangle also has matching diagonals
  10. 9 A rhombus crosses at a right angle
  11. 10 One shape, several names
  12. 11 Building one from measurements
  13. 12 Checkpoint: diagonals, names and building
  14. 13 The big picture
  15. 14 Track quiz

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