Two triangles are congruent if they have:
But we don't have to know all three sides and all three angles ...usually three out of the six is enough. |
There are five ways to find if two triangles are congruent: SSS, SAS, ASA, AAS and HL.
1. SSS (side, side, side)
SSS stands for "side, side, side" and means that we have two triangles with all three sides equal.
For example:
is congruent to: |
(See Solving SSS Triangles to find out more)
If three sides of one triangle are equal to three sides of another triangle, the triangles are congruent.
2. SAS (side, angle, side)
SAS stands for "side, angle, side" and means that we have two triangles where we know two sides and the included angle are equal.
For example:
is congruent to: |
(See Solving SAS Triangles to find out more)
If two sides and the included angle of one triangle are equal to the corresponding sides and angle of another triangle, the triangles are congruent.
3. ASA (angle, side, angle)
ASA stands for "angle, side, angle" and means that we have two triangles where we know two angles and the included side are equal.
For example:
is congruent to: |
(See Solving ASA Triangles to find out more)
If two angles and the included side of one triangle are equal to the corresponding angles and side of another triangle, the triangles are congruent.
4. AAS (angle, angle, side)
AAS stands for "angle, angle, side" and means that we have two triangles where we know two angles and the non-included side are equal.
For example:
is congruent to: |
(See Solving AAS Triangles to find out more)
If two angles and the non-included side of one triangle are equal to the corresponding angles and side of another triangle, the triangles are congruent.
5. HL (hypotenuse, leg)
This one applies only to right angled-triangles!
| or |
HL stands for "Hypotenuse, Leg" (the longest side of a right-angled triangle is called the "hypotenuse", the other two sides are called "legs")
It means we have two right-angled triangles with
- the same length of hypotenuse and
- the same length for one of the other two legs.
It doesn't matter which leg since the triangles could be rotated.
For example:
is congruent to: |
(See Pythagoras' Theorem to find out more)
If the hypotenuse and one leg of one right-angled triangle are equal to the corresponding hypotenuse and leg of another right-angled triangle, the two triangles are congruent.
Caution! Don't Use "AAA"
AAA means we are given all three angles of a triangle, but no sides.
This is not enough information to decide if two triangles are congruent!
Because the triangles can have the same angles but be different sizes:
is not congruent to: |
Without knowing at least one side, we can't be sure if two triangles are congruent.
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When triangles are congruent (identical), one triangle can be moved (through one, or more, rigid motions) to coincide with the other triangle. All corresponding sides and corresponding angles will be congruent.
When triangles are congruent, six facts are always true. | |
Corresponding sides are congruent. | Corresponding angles are congruent. |
The good news is that when trying to verify that two triangles congruent , it is not necessary to show that all six of these facts to be true. There are certain ordered combinations of these facts that are sufficient to verify triangles to be congruent. These combinations guarantee that, given these facts, it will be possible to draw triangles which will take on only one shape (be unique), thus insuring congruency.
Methods that Verify (Prove) Triangles Congruent |
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This is an extension of ASA. In ASA, since you know two sets of angles are congruent, you automatically know the third sets are also congruent since there are 180º in each triangle. | |||||
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Once triangles are proven (verified) congruent, the corresponding leftover "parts" that were not used in SSS, SAS, ASA, AAS and HL, are also congruent. |
Methods that DO NOT Prove Triangles Congruent |
The following ordered combinations of the congruent triangle facts will NOT be sufficient to prove triangles congruent. Let's see why these combinations DO NOT work!
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