Triangle Congruence: Finding Missing Data for ABC and DEF with Given Sides 5, 8, 65

Triangle Congruence with Missing Angle Data

What data must be added so that the triangles are congruent?

656565555888555888AAABBBCCCDDDEEEFFF

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Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

What data must be added so that the triangles are congruent?

656565555888555888AAABBBCCCDDDEEEFFF

2

Step-by-step solution

Let's consider that:

DF = AC = 8

DE = AB = 5

8 is greater than 5, therefore the angle DEF is opposite the larger side and is equal to 65 degrees.

That is, the figure we are missing is the angle of the second triangle.

We will examine which angle is opposite the large side AC.

ABC is the angle opposite the larger side AC so it must be equal to 65 degrees.

3

Final Answer

Angle ABC equals 65.

Key Points to Remember

Essential concepts to master this topic
  • SSA Rule: Two sides and non-included angle can determine congruence
  • Technique: Largest angle is opposite longest side: angle ABC = 65°
  • Check: Verify correspondence - DE = AB = 5, DF = AC = 8 ✓

Common Mistakes

Avoid these frequent errors
  • Assuming any angle can equal 65°
    Don't randomly assign the 65° to any angle = wrong correspondence! This ignores the rule that the largest angle must be opposite the longest side. Always identify which angle is opposite the longest side first.

Practice Quiz

Test your knowledge with interactive questions

Look at the triangles in the diagram.

Which of the statements is true?

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FAQ

Everything you need to know about this question

Why can't angle ACB equal 65°?

+

Angle ACB is opposite side AB (length 5), not the longest side AC (length 8). The largest angle must be opposite the longest side, so angle ABC = 65°.

How do I know which sides correspond between the triangles?

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Match the given measurements: DE = AB = 5, DF = AC = 8, and EF = BC. The angle of 65° is opposite the longest side in both triangles.

What if I can't tell which side is longest?

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Compare the numerical values: 8 > 5, so AC (length 8) is the longest side in triangle ABC. The angle opposite AC is angle ABC.

Why is this called SSA congruence?

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SSA means Side-Side-Angle. We have two corresponding sides (5 and 8) and the angle opposite one of them (65°). This can determine congruence when the angle is opposite the longer side.

Could there be other missing data that works?

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No! Since we already have two sides of each triangle, we need the angle between them or the angle opposite the longest side for congruence. Only angle ABC = 65° works here.

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