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Triangles – Congruence Theorems

AAS: two angles and any side

Two angles are given. The side is not between them. Is that enough?

Angles 70° and 60° are given. A side of 8 is next to the 60°, not between the two.
Step 1 of 3
  1. Angles 70° and 60° are given. A side of 8 is next to the 60°, not between the two.
  2. The third angle is filled in: 180° − 70° − 60° = 50°.
  3. The side of 8 now sits between the 60° and 50° corners.

The rule

Yes, if the two angles total under 180°: they give the third.

A triangle's three angles total 180°. So two angles give the third one.

Now the third angle is known. The side of 8 sits between the 60° and 50° corners. That is ASA, and ASA fixes one triangle.

AAS means two angles and a side that is not between them. If two angles total under 180°, AAS fixes one triangle.

A second example has angles 65° and 48°. They total 65 + 48 = 113°. The third angle is 180 − 113 = 67°. Let a side of 9 join the 48° and 67° corners. It sits between two known angles. That is ASA, so one triangle fits.

Slide the second angle. Watch the third angle change.

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This track

  1. 0 Start here
  2. 1 SAS: two sides and the angle between
  3. 2 SSA can give two triangles
  4. 3 RHS: a right angle, the hypotenuse, a leg
  5. 4 AAS: two angles and any side
  6. 5 Equal sides give equal base angles
  7. 6 Checkpoint: SAS, SSA, RHS and AAS
  8. 7 Equal angles give equal sides
  9. 8 One counter-example ends a claim
  10. 9 Matching parts decide congruence
  11. 10 The apex bisector does three jobs
  12. 11 Checkpoint: converse, counter-example and bisector
  13. 12 The big picture
  14. 13 Track quiz

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