Triangle Congruence Theorem: Proving Two Triangles with Sides 5, 7, and 39° Are Congruent

Triangle Congruence with Mismatched Angle Position

Are the triangles in the image congruent?

If so, according to which theorem?

393939393939555777777555

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Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

Are the triangles in the image congruent?

If so, according to which theorem?

393939393939555777777555

2

Step-by-step solution

Although the lengths of the sides are equal in both triangles, we observe that in the right triangle the angle is adjacent to the side whose length is 7, while in the triangle on the left side the angle is adjacent to the side whose length is 5.

Since it's not the same angle, the angles between the triangles do not match and therefore the triangles are not congruent.

3

Final Answer

No.

Key Points to Remember

Essential concepts to master this topic
  • SAS Rule: Side-Angle-Side requires angle to be between the two given sides
  • Position Matters: 39° adjacent to side 5 versus 39° adjacent to side 7 changes everything
  • Check: Verify angle position matches the same corresponding sides in both triangles ✓

Common Mistakes

Avoid these frequent errors
  • Assuming equal sides and angles guarantee congruence
    Don't think triangles are congruent just because they have the same side lengths and angle measures = wrong conclusion! The angle must be between the same pair of sides in both triangles. Always check that the angle is positioned between the correct corresponding sides for SAS.

Practice Quiz

Test your knowledge with interactive questions

Determine whether the triangles DCE and ABE congruent?

If so, according to which congruence theorem?

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FAQ

Everything you need to know about this question

Why aren't these triangles congruent if they have the same sides and angle?

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The position of the angle matters! In one triangle, the 39° angle is between sides 5 and 7, but in the other triangle, the 39° angle is between different sides. For SAS congruence, the angle must be between the same pair of corresponding sides.

What's the difference between SAS and just having equal parts?

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SAS (Side-Angle-Side) is very specific: you need two sides and the included angle between them. Just having equal parts isn't enough - the angle must be sandwiched between the two equal sides in both triangles.

How can I tell which sides the angle is between?

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Look at the vertex where the angle is located. The two sides that meet at that vertex are the sides that form the angle. In this problem, trace from the 39° angle to see which two sides it connects.

Could these triangles be congruent by a different theorem?

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Let's check! We don't have all three sides (SSS), we don't have two angles and a side (ASA or AAS), so no other congruence theorem applies here. The angle positioning prevents any congruence.

What would make these triangles congruent?

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To be congruent by SAS, the 39° angle would need to be between the same pair of sides in both triangles. For example, both angles should be between the sides of length 5 and 7.

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