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Geometric Twins

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Two tiffin lids swap boxes. Each lid still closes the other box exactly.

Same tick count, same length. Same arc count, same angle.

What this track covers

Your textbook calls these figures Geometric Twins. The maths word is congruent. It means the same shape and the same size.

  • Landing one shape on another. The test for twins.
  • Slide, turn, flip. Three moves that change nothing.
  • A bigger copy. It is not a twin.
  • One line, one corner. The test on a single length or angle.
  • Three sides (SSS). Enough to fix a triangle. Only one shape and size can be drawn.
  • Two sides and the angle between (SAS). Also enough.
  • Two angles and a side (ASA, AAS). Also enough.
  • Three angles (AAA). Not enough.
  • Writing it in order. ΔABC ≅ ΔXYZ, and which corners match.
  • A right angle, the hypotenuse, a side (RHS). Also enough.
  • Equal sides, equal angles. Two equal sides give two equal angles.
  • Twins in a figure. Read the marks, then name the condition.

This is Chapter 1 of Part II, Geometric Twins.

Quick catch-up

  • Fold and match, from symmetry.
  • Turning a shape, from symmetry.
  • Exact, not neat, from constructions.
  • Measuring an angle, from lines and angles.

Jump to lesson

This track

  1. 0 Start here
  2. 1 Landing one on the other
  3. 2 Slide it, turn it, flip it
  4. 3 A bigger copy is a different figure
  5. 4 One line, one corner
  6. 5 Three sides settle it
  7. 6 Two sides and the corner between them
  8. 7 Two corners and a side
  9. 8 When shape is not enough
  10. 9 Writing a congruence in order
  11. 10 RHS: a right angle, the hypotenuse, a side
  12. 11 Equal sides, equal angles
  13. 12 Twins hiding in a figure
  14. 13 Checkpoint: order, RHS and angles
  15. 14 The big picture
  16. 15 Track quiz

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