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SSS Triangle Congruence

Three sets of equal side lengths determine congruence.

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Practice SSS Triangle Congruence

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My SSS Triangle Congruence

What if you were given two triangles and provided with information only about their side lengths? How could you determine if the two triangles were congruent? After completing this concept, you'll be able to use the Side-Side-Side (SSS) shortcut to prove triangle congruency.

Guidance

If 3 sides in one triangle are congruent to 3 sides in another triangle, then the triangles are congruent.

BC¯¯¯¯¯¯¯¯YZ¯¯¯¯¯¯¯, AB¯¯¯¯¯¯¯¯XY¯¯¯¯¯¯¯¯\begin{align*}\overline{BC} \cong \overline{YZ}, \ \overline{AB} \cong \overline{XY}\end{align*}, and AC¯¯¯¯¯¯¯¯XZ¯¯¯¯¯¯¯¯\begin{align*}\overline{AC} \cong \overline{XZ}\end{align*} then \begin{align*}\triangle ABC \cong \triangle XYZ\end{align*}.

This is called the Side-Side-Side (SSS) Postulate and it is a shortcut for proving that two triangles are congruent. Before, you had to show 3 sides and 3 angles in one triangle were congruent to 3 sides and 3 angles in another triangle. Now you only have to show 3 sides in one triangle are congruent to 3 sides in another.

Example A

Write a triangle congruence statement based on the picture below:

From the tic marks, we know \begin{align*}\overline{AB} \cong \overline{LM}, \ \overline{AC} \cong \overline{LK}, \ \overline{BC} \cong \overline{MK}\end{align*}. From the SSS Postulate, the triangles are congruent. Lining up the corresponding sides, we have \begin{align*}\triangle ABC \cong \triangle LMK\end{align*}.

Don’t forget ORDER MATTERS when writing congruence statements. Line up the sides with the same number of tic marks.

Try the following example:

Problem 1:

Is the pair of triangles congruent? If so, write the congruence statement and why.



Example B

Write a two-column proof to show that the two triangles are congruent.

Given: \begin{align*}\overline{AB} \cong \overline{DE}\end{align*}

\begin{align*}C\end{align*} is the midpoint of \begin{align*}\overline{AE}\end{align*} and \begin{align*}\overline{DB}\end{align*}.

Prove: \begin{align*}\triangle ACB \cong \triangle ECD\end{align*}

Statement Reason

1. \begin{align*}\overline{AB} \cong \overline{DE}\end{align*}

\begin{align*}C\end{align*} is the midpoint of \begin{align*}\overline{AE}\end{align*} and \begin{align*}\overline{DB}\end{align*}

1.Given
2. \begin{align*}\overline{AC} \cong \overline{CE}, \ \overline{BC} \cong \overline{CD}\end{align*} 2.Definition of a midpoint
3. \begin{align*}\triangle ACB \cong \triangle ECD\end{align*} 3.SSS Postulate

Note that you must clearly state the three sets of sides are congruent BEFORE stating the triangles are congruent.

Try the following example:

2. Fill in the blanks in the proof below.

Given: \begin{align*}\overline{AB} \cong \overline{DC}, \ \overline{AC} \cong \overline{DB}\end{align*}

Prove: \begin{align*}\triangle ABC \cong \triangle DCB\end{align*}

Statement Reason
1. 1.
2. 2. Reflexive PoC
3. \begin{align*}\triangle ABC \cong \triangle DCB\end{align*} 3.

1. The triangles are congruent because they have three pairs of sides congruent. \begin{align*} \triangle DEF \cong \triangle IGH\end{align*}.

2.

Statement Reason
1. \begin{align*}\overline{AB} \cong \overline{DC}, \ \overline{AC} \cong \overline{DB}\end{align*} 1. Given
2. \begin{align*}\overline{BC} \cong \overline{CB}\end{align*} 2. Reflexive PoC
3. \begin{align*}\triangle ABC \cong \triangle DCB\end{align*} 3. SSS Postulate

HOMEWORK:

Are the pairs of triangles congruent? If so, write the congruence statement and why.

State the additional piece of information needed to show that each pair of triangles is congruent.

1. Use SSS
2. Use SSS

Fill in the blanks in the proofs below.

1. Given: \begin{align*}B\end{align*} is the midpoint of \begin{align*}\overline{DC}\end{align*} \begin{align*}\overline{AD} \cong \overline{AC}\end{align*} Prove: \begin{align*}\triangle ABD \cong \triangle ABC\end{align*}
Statement Reason
1. 1.
2. 2. Definition of a Midpoint
3. 3. Reflexive PoC
4. \begin{align*}\triangle ABD \cong \triangle ABC\end{align*} 4.

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