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Geometric Twins

Two sides and the corner between them

Two lengths and an angle. Is that enough for a triangle?

Flip the switch. Then slide the length.

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Angle between the sides: one triangle. 45 mm side opposite the angle: two.

The angle must sit between

Yes, if the angle is between the two sides. Then only one triangle fits.

SAS: two sides and the angle between them give one triangle. S is a side and A is an angle. The angle between the sides is the included angle.

Now move the angle so it is not between the sides. This is SSA: side, side, angle. The same numbers can now make two triangles, one triangle, or none.

One 35° angle, one 60 mm side
Where the other length goesTriangles that fit
A 45 mm side, angle betweenExactly one
A 45 mm side opposite the angleTwo. Third sides: 78.1 and 20.2 mm
A 30 mm side opposite the angleNone. It cannot reach
A 70 mm side opposite the angleExactly one. It is longer than the 60 mm side

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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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