CPCTC ⭐ Definition, Postulates, Theorem, Proof, Examples (2023)

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Welcome to Brighterly, where learning mathematics is an exciting adventure! Today, we invite you to delve into the captivating world of geometry as we explore the concept of CPCTC – Corresponding Parts of Congruent Triangles are Congruent. At Brighterly, we believe in making math accessible, engaging, and tailored for young minds.

In this article, we will embark on a journey through the definition, postulates, theorem, proof, and examples of CPCTC. By understanding this fundamental principle, our young mathematicians at Brighterly will develop a solid foundation in geometry, logical reasoning, and problem-solving skills.

Geometry, with its intricate shapes and spatial relationships, plays a vital role in numerous real-life applications, from designing buildings to creating stunning works of art. That’s why it’s crucial for our students to grasp the essence of CPCTC, which establishes the congruence of corresponding parts in congruent triangles.

What Is CPCTC in Geometry?

Geometry is a fascinating area of mathematics that involves shapes, sizes, and properties of space. One of the most interesting concepts in geometry is CPCTC. If you’re wondering, ‘What does CPCTC stand for?’, it’s an abbreviation that stands for Corresponding Parts of Congruent Triangles are Congruent. In simpler terms, if two triangles are congruent (exactly the same in size and shape), then their corresponding parts (sides and angles) are also congruent.

Understanding CPCTC can be a big step in your journey to mastering geometry. It is used in various real-life scenarios such as architecture, engineering, and even in art. So, let’s delve deeper and understand the concept of CPCTC.

CPCTC: Definition

CPCTC is a principle in geometry that affirms that if two triangles are congruent, then their corresponding parts – the sides and angles that match – are also congruent. This principle is widely used in proofs and problem-solving in geometry. Imagine two identical triangle-shaped cookies. The corresponding sides (the equal lengths) and angles (the equal corners) of these cookies would be exactly the same, and that’s precisely what CPCTC is all about!

Congruent Triangles

In geometry, congruent triangles are triangles that are identical in terms of size and shape. If you were to place one triangle on top of the other, they would match perfectly. The concept of congruent triangles is fundamental in understanding CPCTC. For two triangles to be congruent, they must satisfy one of the following conditions: Side-Side-Side (SSS), Side-Angle-Side (SAS), Angle-Side-Angle (ASA), Angle-Angle-Side (AAS), or Right angle-Hypotenuse-Side (RHS) congruence rules.

Corresponding Parts

The term ‘corresponding parts’ refers to the sides and angles in a triangle that occupy the same relative position. In congruent triangles, the corresponding parts are equal in measurement. It means that the first angle of one triangle is congruent to the first angle of the other, the second to the second, and so on. Similarly, the sides of the triangles that correspond are also equal in length. This is a crucial concept to understand when learning about CPCTC.

CPCTC Triangle Congruence

CPCTC is a powerful tool that helps us in proving the congruence of parts of two triangles, given that the triangles themselves are congruent. This concept is derived from the principles of triangle congruence mentioned above (SSS, SAS, ASA, AAS, RHS). Once we have established the congruence of the triangles using any of these rules, we can assert that their corresponding parts are congruent by CPCTC.


The proof of CPCTC is based on the postulates and theorems of geometry. Here’s how it works:

Suppose we have two triangles, Triangle ABC and Triangle DEF. We know that they are congruent (using one of the congruence rules). By definition of congruence, the corresponding parts of these triangles are congruent. Therefore, we can say that ∠A ≅ ∠D, ∠B ≅ ∠E, ∠C ≅ ∠F, AB ≅ DE, BC ≅ EF, and AC ≅ DF.

Solved Examples on CPCTC

Let’s apply CPCTC in practice with the following example:

Example: In two congruent triangles ABC and DEF, ∠A = 40°, ∠B = 80°, and ∠C = 60°. Find the measures of ∠D, ∠E, and ∠F.

Solution: Since the triangles are congruent, by CPCTC, the corresponding angles of the triangles will be congruent. Therefore, ∠D = ∠A = 40°, ∠E = ∠B = 80°, and ∠F = ∠C = 60°.

This example demonstrates how we can use CPCTC to solve problems involving congruent triangles.

Practice Problems on CPCTC

To further reinforce your understanding of CPCTC, here are some practice problems:

  1. If triangle ABC is congruent to triangle DEF and AB = 7 cm, BC = 8 cm, and AC = 9 cm, find the lengths of DE, EF, and DF.

  2. In two congruent triangles PQR and STU, ∠P = 35° and ∠Q = 55°. Determine the measures of ∠S and ∠T.

Remember, practice is the key to mastering any concept, especially when it comes to complex geometrical principles like CPCTC.


In conclusion, CPCTC (Corresponding Parts of Congruent Triangles are Congruent) is a vital concept within the realm of geometry that holds immense value for our young mathematicians at Brighterly. By understanding CPCTC, our students unlock the ability to analyze the congruence of triangles and deduce the congruence of their corresponding parts.

Through this journey into the world of CPCTC, we empower our students to develop critical thinking skills, logical reasoning, and problem-solving abilities. By delving into the definition, postulates, theorem, and proof of CPCTC, our students gain a comprehensive understanding of the interconnections between congruent triangles and their corresponding parts.

At Brighterly, we believe in making mathematics enjoyable and accessible. By incorporating CPCTC into our curriculum, we foster a deep appreciation for the beauty and practical applications of geometry. From architecture to engineering, our young learners discover the significance of CPCTC in various real-world contexts.

Frequently Asked Questions on CPCTC

What does CPCTC stand for?

CPCTC stands for “Corresponding Parts of Congruent Triangles are Congruent.” This acronym highlights the principle that when two triangles are congruent, their corresponding parts – angles and sides in the same positions – are also congruent. This concept serves as a powerful tool in geometry to establish relationships between congruent triangles.

How is CPCTC used in geometry?

CPCTC plays a crucial role in proving congruence and solving problems in geometry. By applying CPCTC, we can make assertions about the equality of corresponding parts when two triangles are known to be congruent. For example, if we have proven that two triangles are congruent using the SAS (Side-Angle-Side) criterion, we can confidently state that their corresponding sides and angles are congruent.

What are congruent triangles?

Congruent triangles are triangles that are identical in both shape and size. When two triangles are congruent, all corresponding angles and sides have equal measures. This means that if you were to superimpose one congruent triangle onto another, they would perfectly overlap.

How can I prove that two triangles are congruent?

There are several methods to prove the congruence of triangles. These include the Side-Side-Side (SSS), Side-Angle-Side (SAS), Angle-Side-Angle (ASA), Angle-Angle-Side (AAS), and Right angle-Hypotenuse-Side (RHS) congruence rules. To prove congruence, you need to show that the corresponding elements of the triangles satisfy one of these criteria.

Why is CPCTC important in geometry?

CPCTC is a fundamental concept in geometry that allows us to make logical connections between congruent triangles. It provides a systematic approach to deducing the equality of corresponding parts, which is vital for solving geometric problems and proving theorems. Understanding CPCTC enables students to analyze the relationships between congruent triangles and apply their knowledge to various geometric scenarios.

Can CPCTC be used in real-life applications?

Yes, CPCTC finds practical applications in real-life situations. For example, architects and engineers use CPCTC to ensure the accuracy and consistency of corresponding elements in structures. By applying the principles of CPCTC, they can ensure that congruent components of a building or object fit together seamlessly.

How can I practice applying CPCTC?

To practice applying CPCTC, you can work through a variety of geometry problems and proofs that involve triangle congruence. Start with basic examples and gradually progress to more complex scenarios. Engaging in hands-on activities, such as constructing congruent triangles and analyzing their corresponding parts, can also enhance your understanding of CPCTC.

Sources of Information
  1. CPCTC – Wikipedia
  2. CPCTC – Regents Prep
  3. Congruent Triangles and CPCTC – GeoGebra


What is an example of a Cpctc proof? ›


For example, Consider triangles ABC and CDE in which BC = CD and AC = CD are given. Now the two triangles are congruent, therefore, using CPCTC, AB = DE, ∠ABC = ∠EDC and ∠BAC = ∠DEC.

What is an example of a postulate and theorem? ›

If two planes intersect, then their intersection is a line (Postulate 6). A line contains at least two points (Postulate 1). If two lines intersect, then exactly one plane contains both lines (Theorem 3). If a point lies outside a line, then exactly one plane contains both the line and the point (Theorem 2).

Is Cpctc a theorem or postulate? ›

What is the CPCTC theorem? The CPCTC theorem states that if two or more triangles are congruent to each other, then the corresponding angles and the sides of the triangles are also congruent to each other.

What is an example of proving SSS postulate? ›

What is an example of the SSS postulate or theorem? The SSS postulate applies to triangles that have the same measurements for corresponding sides. An example would be a triangle that has side lengths 3, 4, and 5 and a triangle that has side lengths 4, 3, and 5.

What postulate proves these triangles congruent? ›

Side-Angle-Side Postulate

This is called the Side-Angle-Side (SAS) Postulate and it is a shortcut for proving that two triangles are congruent. The placement of the word Angle is important because it indicates that the angle you are given is between the two sides.

How do you prove theorems involving triangles and congruent triangles? ›

The simplest way to prove that triangles are congruent is to prove that all three sides of the triangle are congruent. When all the sides of two triangles are congruent, the angles of those triangles must also be congruent. This method is called side-side-side, or SSS for short.

What are 3 example of postulates? ›

Some examples of mathematical postulates are: A line grows infinitely. All right angles are equivalent. The intersection between two planes is a line.

What are 5 examples of postulates in geometry? ›

Postulate 1 : A straight line may be drawn from any one point to Any other point. Postulate 2 : A terminated line can be produced indefinitely. Postulate 3 : A circle can be drawn with any centre and any radius. Postulate 4 : All right angles are equal to one another.

What is one example of theorem? ›

In geometry, a proposition is commonly considered as a problem (a construction to be effected) or a theorem (a statement to be proved). The statement “If two lines intersect, each pair of vertical angles is equal,” for example, is a theorem.

What is a postulate a theorem? ›

Postulates are statements that are accepted as true without being proven. Theorems are statements that can be proven. Postulates are generally the starting point for proving theorems.

What are the 8 postulate? ›

Postulate 8: Through any three noncollinear points there exists exactly one plane. Postulate 9: A plane contains at least three noncollinear points. Postulate 10: If two points lie in a plane, then the line containing them lies in the plane. Postulate 11: If two planes intersect, then their intersection is a line.

Can you prove a postulate? ›

Answer and Explanation: No, a postulate does not require proof. A more technical definition of a postulate in math is a statement that is generally accepted as true with or without a proof indicating as such. By this definition, we see that postulates are accepted as true whether they have a proof or not.

Is it possible to prove a postulate? ›

Thus a postulate is a hypothesis advanced as an essential presupposition to a train of reasoning. Postulates themselves cannot be proven, but since they are usually self-evident, their acceptance is not a problem. Here is a good example of a postulate (given by Euclid in his studies about geometry).

What are the 5 theorems of postulates that prove triangle congruence? ›

Congruent Triangle Theorem and Postulates
  • Angle-Angle-Side Theorem (AAS theorem)
  • Hypotenuse-Leg Theorem (HL theorem)
  • Side-Side-Side Postulate (SSS postulate)
  • Angle-Side-Angle Postulate (ASA postulate)
  • Side-Angle-Side Postulate (SAS postulate)
Mar 14, 2012

How do you find the postulate of a triangle? ›

Side-Angle-Side (SAS) Postulate

If two sides and the included angle of one triangle are congruent to two sides and the included angle of another triangle, then the two triangles are congruent.

What are 5 ways to prove a triangle? ›

What are the 5 ways to prove triangles congruent? There are five theorems that can be used to show that two triangles are congruent: the Side-Side-Side (SSS) theorem, the Side-Angle-Side (SAS) theorem, the Angle-Angle-Side (AAS) theorem, the Angle-Side-Angle (ASA) theorem, and the Hypotenuse-Leg (HL) theorem.

What are the 4 ways to prove triangles are congruent? ›

Conditions for Congruence of Triangles:
  • SSS (Side-Side-Side)
  • SAS (Side-Angle-Side)
  • ASA (Angle-Side-Angle)
  • AAS (Angle-Angle-Side)
  • RHS (Right angle-Hypotenuse-Side)

What are the 5 theorems of a triangle? ›

The Side–Side–Side Theorem, Side Angle Side Theorem, Angle Side Angle Theorem, Angle-Angle Side Theorem, and Right angle-Hypotenuse-Side or the Hypotenuse Leg Theorem are the five triangle congruence theorems.

What are the first 5 postulates? ›

The five postulates on which Euclid based his geometry are:
  • To draw a straight line from any point to any point.
  • To produce a finite straight line continuously in a straight line.
  • To describe a circle with any center and distance.
  • That all right angles are equal to one another.

What are the 4 postulate? ›

As originally stated, the four criteria are: (1) The microorganism must be found in diseased but not healthy individuals; (2) The microorganism must be cultured from the diseased individual; (3) Inoculation of a healthy individual with the cultured microorganism must recapitulated the disease; and finally (4) The ...

What are the 10 postulate? ›

Postulate 10: (Parallel Postulate) Through a point not on a line, exactly one line is parallel to the given line. Postulate 11: (Corresponding Angles Postulate) If two parallel lines are cut by a transversal, then the corresponding angles are congruent.

What is a proof in geometry? ›

Geometric proofs are a list of Statements and Reasons used to prove that a given mathematical concept or idea is true. Statements are claims about a geometric problem that cannot be proven true until backed by a mathematical Reason. Reasons are pieces of evidence that support a Statement.

How many postulates are there? ›

Postulates in geometry is very similar to axioms, self-evident truths, and beliefs in logic, political philosophy, and personal decision-making. The five postulates of Euclidean Geometry define the basic rules governing the creation and extension of geometric figures with ruler and compass.

What is a three point postulate? ›

The 3 Point Postulate: Through any three non-collinear points, there exists exactly one plane. Plane-Point Postulate: A plane contains at least three non-collinear points.

What are the 12 theorems? ›

The geometry theorems are: Isosceles Triangle Theorem, Angle Sum Triangle Theorem, Equilateral Triangle Theorem, Opposite Angle Theorem, Supplementary Angle Theorem, Complementary Angle Theorem, 3 Parallel Line Theorems, Exterior Angle Theorem, Exterior Angles of a Polygon and Interior Angles of a Polygon.

What are the 3 types of theorem? ›

Linear Pair Theorem If two angles form a linear pair, then they are supplementary. supplements theorem If two angles are supplements of the same angle, then they are congruent. Congruent complements theorem If two angles are complements of the same angle, then they are congruent.

What are the three theorems? ›

This section covers three theorems of fundamental importance to the topic of differential calculus: The Extreme Value Theorem, Rolle's Theorem, and the Mean Value Theorem.

What does Cpctc stand for quizlet? ›

Terms in this set (14) What does CPCTC mean? corresponding parts of congruent triangles are congruent. Why must you show in your proof before using cpctc? two triangles are congruent.

Is AAA a congruence theorem? ›

Knowing only angle-angle-angle (AAA) does not work because it can produce similar but not congruent triangles. When you're trying to determine if two triangles are congruent, there are 4 shortcuts that will work.

How do you prove lines are parallel? ›

If two lines are cut by a transversal so the corresponding angles are congruent, then the lines are parallel. If two lines are cut by a transversal so the alternate interior angles are congruent, then the lines are parallel.

Does Cpctc work for parallelograms? ›

Since the opposite sides of the parallelogram are corresponding parts of the congruent triangles, we can use CPCTC to show that they must be congruent.

Can you use Cpctc for similar triangles? ›

' You often use CASTC in a proof immediately after proving triangles similar (in precisely the same way that you use CPCTC after proving triangles congruent).

What is 1st postulate? ›

The first postulate of special relativity is the idea that the laws of physics are the same and can be stated in their simplest form in all inertial frames of reference.

What is a postulate answer? ›

: to assume or claim as true, existent, or necessary : depend upon or start from the postulate of.

What is a theorem in math? ›

Theorems are what mathematics is all about. A theorem is a statement which has been proved true by a special kind of logical argument called a rigorous proof.

What is 5 postulate in short? ›

If a straight line falling on two straight lines makes the interior angles on the same side of it taken together less than two right angles, then the two straight lines, if produced indefinitely, meet on that side on which the sum of angles is less than two right angles.

Is a postulate a fact? ›

A postulate is an assumption, that is, a proposition or statement, that is assumed to be true without any proof .

Which statement is a theorem? ›

A theorem is a statement that can be demonstrated to be true by accepted mathematical operations and arguments. In general, a theorem is an embodiment of some general principle that makes it part of a larger theory. The process of showing a theorem to be correct is called a proof.

What is an example of a postulate in math? ›

In geometry, a postulate is a statement that is assumed to be true based on basic geometric principles. An example of a postulate is the statement "exactly one line may be drawn through any two points." A long time ago, postulates were the ideas that were thought to be so obviously true they did not require proof.

Is the 5th postulate proven? ›

Euclid's fifth postulate cannot be proven as a theorem, although this was attempted by many people. Euclid himself used only the first four postulates ("absolute geometry") for the first 28 propositions of the Elements, but was forced to invoke the parallel postulate on the 29th.

What are the proofs in geometry? ›

Two-column, paragraph, and flowchart proofs are three of the most prevalent proofs used in geometry. Each presents the steps and the Statement-and-Justification pairs in different ways.

What is the base angles theorem? ›

Base Angles Theorem

If two sides of a triangle are congruent, then the angles opposite them are congruent.

What are the 5 valid methods for proving triangle congruence? ›

There are five ways to find if two triangles are congruent: SSS, SAS, ASA, AAS and HL.
  • SSS (side, side, side) SSS stands for "side, side, side" and means that we have two triangles with all three sides equal. ...
  • SAS (side, angle, side) ...
  • ASA (angle, side, angle) ...
  • AAS (angle, angle, side) ...
  • HL (hypotenuse, leg)

Can Cpctc prove congruent angles? ›

CPCTC is commonly used at or near the end of a proof which asks the student to show that two angles or two sides are congruent.

Does Cpctc prove triangles congruent? ›

CPCTC stands for "corresponding parts of congruent triangles are congruent" and tells us if two or more triangles are congruent, then their corresponding angles and sides are congruent as well.

What must you show before using Cpctc in a proof? ›

BEFORE YOU USE CPCTC YOU MUST PROVE THAT THE TRIANGLES IN QUESTION ARE CONGRUENT FIRST!!! Let's analyze a couple of these, and then we will get to practicing... AABC ADEF by some congruence property.

What is the 45 45 90 rule? ›

The 45-45-90 triangle rule states that the three sides of the triangle are in the ratio 1:1:\(\sqrt{2}\). So, if the measure of the two congruent sides of such a triangle is x each, then the three sides will be x, x and \(\sqrt{2}x\). This rule can be proved by applying the Pythagorean theorem.

What is the formula for 45 45 90 triangle? ›

45°−45°−90° triangle is a commonly encountered right triangle whose sides are in the proportion 1:1:√2 . The measures of the sides are x , x , and x√2 . In a 45°−45°−90° triangle, the length of the hypotenuse is √2 times the length of a leg.


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