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Congruent triangles are triangles that are identical to each other, having three equal sides and three equal angles.^{[1] X Research source } Writing a proof to prove that two triangles are congruent is an essential skill in geometry. Since the process depends upon the specific problem and givens, you rarely follow exactly the same process. This can be frustrating; however, there is an overall pattern to solving geometric proofs and there are specific guidelines for proving that triangles are congruent. Once you know them, you’ll be able to prove them on your own with ease.
Steps
Part 1
Part 1 of 2:Proving Congruent Triangles
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1Draw a diagram. A diagram may already be provided, but if one is not, it’s essential to draw one. Try to draw it as accurately as you can. Include all of the given information in your diagram. If two sides or angles are congruent (equal), mark them as such.^{[2] X Research source }
 It may be beneficial to sketch a first diagram that is not accurate and redraw it a second time to look better.
 If your diagram has two overlapping triangles, try redrawing them as separate triangles. It will be much easier to find and mark the congruent pieces.
 If your diagram does not have two triangles, you might have a different kind of proof. Double check to make sure the problem asks you to prove congruency of two triangles.

2Identify the known information. Using the givens and your knowledge of geometry, you can start to prove some things and determine if any sides and/or angles of two triangles are congruent. Think about the parts of the proof logically and determine stepbystep how to get from the givens to the final conclusion.^{[3] X Research source }
 For example: Using the following givens, prove that triangle ABC and CDE are congruent: C is the midpoint of AE, BE is congruent to DA. If C is the midpoint of AE, then AC must be congruent to CE because of the definition of a midpoint. This allows you prove that at least one of the sides of both of the triangles are congruent.
 If BE is congruent to DA then BC is congruent to CD because C is also the midpoint of AD. You now have two congruent sides.
 Also, because BE is congruent to DA, angle BCA is congruent to DCE because vertical angles are congruent.
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3Choose the correct theorem to prove congruency. There are five theorems that can be used to prove that triangles are congruent. Once you have identified all of the information you can from the given information, you can figure out which theorem will allow you to prove the triangles are congruent.^{[4] X Research source }
 Sidesideside (SSS): both triangles have three sides that equal to each other.
 Sideangleside (SAS): two sides of the triangle and their included angle (the angle between the two sides) are equal in both triangles.
 Anglesideangle (ASA): two angles of each triangle and their included side are equal.
 Angleangleside (AAS): two angles and a nonincluded side of each triangle are equal.
 Hypotenuse leg (HL): the hypotenuse and one leg of each triangle are equal. This only applies to right triangles.
 For example: Because you were able to prove that two sides with their included angle were congruent, you would use sideangleside to prove that the triangles are congruent.
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Part 2
Part 2 of 2:Writing a Proof
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1Set up a twocolumn proof. The most common way to set up a geometry proof is with a twocolumn proof. Write the statement on one side and the reason on the other side. Every statement given must have a reason proving its truth. The reasons include it was given from the problem or geometry definitions, postulates, and theorems.^{[5] X Research source }

2Write down the givens. The easiest step in the proof is to write down the givens. Write the statement and then under the reason column, simply write given. You can start the proof with all of the givens or add them in as they make sense within the proof.^{[6] X Research source }
 Write down what you are trying to prove as well. If you want to prove that triangle ABC is congruent to XYZ, write that at the top of your proof. This will also be the conclusion of your proof.

3Use the appropriate theorems, definitions, and postulates as reasons. When developing a proof, you need a solid foundation in geometry before you can begin. Knowing the relevant theorems, definitions, and postulates is essential. A working knowledge of these will help you to find reasons for your proof.^{[7] X Research source }
 Some good definitions and postulates to know involve lines, angles, midpoints of a line, bisectors, alternating and interior angles, etc.
 You cannot prove a theorem with itself. If you're trying to prove that base angles are congruent, you won't be able to use "Base angles are congruent" as a reason anywhere in your proof.

4Order the proof logically. When constructing a proof, you want to think through it logically. Try to order all of your steps so that they naturally follow each other. Sometimes it helps to work the problem backwards: start with the conclusion and work your way back to the first step.^{[8] X Research source }
 Every step must be included even if it seems trivial.
 Read through the proof when you are done to check to see if it makes sense.
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Community Q&A

QuestionIn sss, are the 3 sides congruent?DonaganTop AnswererYes, you can prove congruency if you can show that each of the three sides of a triangle is congruent (equal in length) respectively to a side of the other triangle.

QuestionMy teacher will never give marks if I follow these steps. He just wants exactly the same written in classwork. If I solve at least half, and it's correct, teachers are supposed to give marks but our teacher will give a 0. What do I do?DonaganTop AnswererGive your teacher what s/he wants. You won't have to put up with that forever.

QuestionWhat do I write if all three sides are not congruent when doing a geometry proof?Community AnswerIt will always be a congruent if you are to prove any (angle/Side) provided you take the right triangle.

QuestionWhat do I do when asked to prove congruency when given two angles and a side?DonaganTop AnswererAssuming the given side is between the two given angles, that's ASA ("anglesideangle"), which is in itself proof of congruency. If the given side is not between the given angles, you would need additional information in order to prove congruency.

QuestionAre SAS and AAS axioms the same?DonaganTop AnswererNo, they're not the same, but they are of equal value in proving triangle congruency.

QuestionWhat can I do if I can't do the questions of proof from out of the textbook?Community AnswerKeep trying. You can do it or you can otherwise have a look through some of the worksheets available online.

QuestionWhat is the difference between corresponding sides and given?DonaganTop Answerer"Corresponding sides" (as a reason in a proof of congruency) means that sides occupying the same position in congruent polygons (triangles in this case) are congruent (or equal in length). "Given" means that the information you are presenting is true by definition or by earlier proof.
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Tips
 If your givens include the word "perpendicular," do not say that an angle is 90 degrees due to definition of perpendicular lines. Instead, write a statement saying such angle is a right angle because of "definition of perpendicular lines" and then write another statement saying said angle is 90 degrees because of "definition of right angle."Thanks!
References
 ↑ http://www.mathsisfun.com/geometry/trianglescongruentfinding.html
 ↑ https://www.wyzant.com/resources/lessons/math/geometry/introduction/two_column_proofs
 ↑ http://www.ck12.org/geometry/TwoColumnProofs/lesson/TwoColumnProofsBSCGEOM/
 ↑ http://www.mathsisfun.com/geometry/trianglescongruentfinding.html
 ↑ https://www.wyzant.com/resources/lessons/math/geometry/introduction/two_column_proofs
 ↑ https://www.wyzant.com/resources/lessons/math/geometry/introduction/two_column_proofs
 ↑ http://www.ck12.org/geometry/TwoColumnProofs/lesson/TwoColumnProofsBSCGEOM/
 ↑ https://www.wyzant.com/resources/lessons/math/geometry/introduction/two_column_proofs
About This Article
To write a congruent triangles geometry proof, start by setting up 2 columns with “Statements” on the left and “Reasons” on the right. Then, write known information as statements and write “Given” for their reasons. Next, write the rest of the statements you have to prove on the left, and write the corresponding theorems, definitions, and postulates you need to explain those statements on the right. Be sure to think through all the steps in your proof and order them logically so every statement leads to the one that follows until you get to your conclusion. To learn how to prove congruent triangles, keep reading!