CPCTC
CPCTC is an acronym for "corresponding parts of congruent triangles are congruent'.
When two or more triangles are congruent then their corresponding sides and angles are congruent.
Lesson Plan
What is CPCTC?
The CPCTC theorem states that when two triangles are congruent, then their corresponding parts are equal.
When \(\triangle \text{ABC}\cong\triangle{DEF}\), the three pairs of sides and three pairs of angles of \(\triangle \text{ABC}\) = the three pairs of corresponding sides and three pairs of corresponding angles of \(\triangle \text{DEF}\).
Congruent Triangles
Two triangles are said to be congruent if they have exactly the same size and the same shape.
The equal sides and angles may not be in the same position.
Corresponding Parts
Corresponding sides mean the three sides in one triangle are in the same position or spot as in the other triangle.
Corresponding angles mean the three angles in one triangle are in the same position or spot as in the other triangle.
Explanation
In these two triangles ABC and DEF, let us identify the 6 parts: i.e. the three corresponding sides and the three corresponding angles.
AB corresponds to DE, BC corresponds to EF, CD corresponds to FD.
\(\angle\) A corresponds to\(\angle\)D, \(\angle\)B corresponds to\(\angle\)E, \(\angle\)C corresponds to\(\angle\)F.
And if \(\triangle\text{ABC} \cong \triangle\text{DEF}\), then as per the cpctc theorem, the corresponding sides and angles are equal.
i.e. AB = DE, BC = EF,CD = FD
\(\angle\) A = \(\angle\)D, \(\angle\)B = \(\angle\)E, \(\angle\)C = \(\angle\)F.
Explain CPCTC Triangle Congruence
Example
CPCTC states that if two triangles are congruent by any method, then all ot the corresponding sides and angles are equal.
Consider the two triangles \(\triangle1\) and \(\triangle2\) in each of the cases.
Explanation using CPCTC Worksheet
Criterion  Explanation  CPCTC 
SSS  All the 3 corresponding sides are equal.  All the corresponding angles are also equal. 
SAS  2 corresponding sides and the included angle are equal.  The other corresponding sides and the other 2 corresponding angles are also equal. 
ASA  2 corresponding angles and the included sides are equal.  The other corresponding angles and the other 2 corresponding sides are also equal. 
RHS / HL  The hypotenuse and one leg of one triangle are equal to the corresponding hypotenuse and a leg of the other.  The other corresponding legs and the other two corresponding angles are equal. 
 A related theorem is CPCFC, which is the acronym of congruent parts of congruent figures are congruent which applies to any polygon or polyhedron proven congruent.

Using CPCTC we prove many more theorems on parallelogram. A few are "Both pairs of the opposite sides of a parallelogram are congruent". "Both pairs of opposite angles are congruent." "Is Parallelogram, a rhombus?"
How to Geometrically Prove CPCTC?
Consider triangles ABC and CDE. In your CPCTC worksheets, you can have the twocolumn proof done.
Proof
Statement  Reasons 
BC = CD  Given 
AC = CD  Given 
\(\angle ACB = \angle EDC\)  Vertically Opposite Angles are equal. 
\(\triangle\text{ABC} \cong \triangle\text{DEF}\)  By SAS (sideangleside)criterion 
AB = DE, \(\angle\)ABC = \(\angle\)EDC and \(\angle\)BAC = \(\angle\)DEC  CPCTC 
CPCTC Parallelogram
Example: Theorem: The diagonals of a parallelogram bisect each other.
Explanation using CPCTC Worksheet
Statement  Reason 

AD = CB  \(\because\) opposite sides of parallelogram are equal 
\(\angle\)DCA = \(\angle\) CAB  \(\because\)Alternate interior angles are equal. 
\(\angle\)ABC = \(\angle\) ADC  \(\because\)Alternate interior angles are equal. 
\(\triangle AOD \cong \triangle COB\)  By ASA Criterion 
\(\therefore\)AO = CO , DO = BO  By CPCTC 
 Look for the congruent triangles keeping CPCTC in mind.
 Before using CPCTC, show that the two triangles are congruent.
Solved Examples
Example 1 
Find the length of LM.
Solution
Let us first find the missing angles.
In \(\triangle\) EFG,
\[\begin{align}\angle EGF + \angle GFE + \angle FEG &= 180^{\circ}\\\angle EGF + 30^{\circ}+ 102^{\circ} &= 180^{\circ}\\\angle EGF &= 180^{\circ}  132^{\circ}\\ &= 48^{\circ}\end{align}\]
In \(\triangle\) LMN,
\[\begin{align}\angle LMN + \angle MNL+ \angle NLM &= 180 ^{\circ}\\\angle LMN + 48^{\circ}+ 102^{\circ} &= 180^{\circ}\\\angle LMN &= 180^{\circ}  132^{\circ}\\ &= 30^{\circ}\end{align}\]
Statement  Reasons 
\(\angle\) FEG= \(\angle\) MLN  Given 
\(\angle\) EGF = \(\angle\) MNL  By Triangle sum property 
\(\angle\) GFE = \(\angle\) LMN  By Triangle sum property 
\(\triangle\) EFG = \(\triangle\) LMN  AAA criterion 
LM = EF = 3  CPCTC 
\(\therefore\) LM = 3 units 
Example 2 
Given: PR = RS. Find y.
Solution
Statement  Reason 
PR = RS  Given 
QR = QR  Reflexive property 
PQR = SPR  HL criterion 
PQ = QS  By cpctc 
\[\begin{align} 4y &= 28\\y &= 28 \div4\\ &= 7 \end{align}\]
\(\therefore\)y = 7 units. 
Example 3 
Find angle BAC and angle DEC and b if the triangles ABC and CDE are congruent.
Solution
Triangle BAC and triangle DEC are congruent.
By cpctc, we have \(\angle\) BAC = \(\angle\) DEC
\[\begin{align} \therefore 5x = 3x +18\\2x =18\\x= 9 ^{\circ}\end{align}\]
Substitute 9 in any of the angles.
\[\begin{align}5x = 5\times 9= 45 ^{\circ}\end{align}\]
\(\therefore\) \(\angle\)BAC = \(\angle\) DEC= 45 \(^{\circ}\) 
Interactive Questions
Here are a few activities for you to practice. Select/Type your answer and click the "Check Answer" button to see the result.
Let's Summarize
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Frequently Asked Questions (FAQs)
1. Is CPCTC a theorem?
Yes. CPCTC is a theorem that says corresponding parts of congruent triangles are congruent.
2. What is CPCTC for similar triangles?
Cpctc for similar triangles is not true. Corresponding angles of the two similar triangles are equal, whereas, corresponding sides of the triangles are not equal, but proportional.
3. How do you prove CPCTC?
After showing the proposed triangles are congruent, we can immediately say that the corresponding parts of congruent triangles are congruent.