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Properties of Equality and Congruence

Logical rules that allow equations to be manipulated and solved.

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Properties of Equality and Congruence

What if you wanted to solve an equation and justify each step? What mathematical properties could you use in your justification? After completing this Concept, you be able to see how the properties of equality from Algebra I relate to geometric properties of congruence.

Watch This

CK-12 Foundation: Chapter2PropertiesofEqualityandCongruenceA

James Sousa: Introduction to Proof Using Properties of Equality

Guidance

The basic properties of equality were introduced to you in Algebra I. Here they are again:

For all real numbers \begin{align*}a, b\end{align*}, and @$\begin{align*}c\end{align*}@$:

Examples
Reflexive Property of Equality @$\begin{align*}a = a\end{align*}@$ @$\begin{align*}25 = 25\end{align*}@$
Symmetric Property of Equality @$\begin{align*}a = b\end{align*}@$ and @$\begin{align*}b = a\end{align*}@$ @$\begin{align*}m \angle P = 90^\circ\end{align*}@$ or @$\begin{align*}90^\circ = m \angle P\end{align*}@$
Transitive Property of Equality @$\begin{align*}a = b\end{align*}@$ and @$\begin{align*}b = c\end{align*}@$, then @$\begin{align*}a = c\end{align*}@$ @$\begin{align*}a + 4 = 10\end{align*}@$ and @$\begin{align*}10 = 6 + 4\end{align*}@$, then @$\begin{align*}a + 4 = 6 + 4\end{align*}@$
Substitution Property of Equality If @$\begin{align*}a = b\end{align*}@$, then @$\begin{align*}b\end{align*}@$ can be used in place of @$\begin{align*}a\end{align*}@$ and vise versa. If @$\begin{align*}a = 9\end{align*}@$ and @$\begin{align*}a - c = 5\end{align*}@$, then @$\begin{align*}9 - c = 5\end{align*}@$
Addition Property of Equality If @$\begin{align*}a = b\end{align*}@$, then @$\begin{align*}a + c = b + c\end{align*}@$. If @$\begin{align*}2x = 6\end{align*}@$, then @$\begin{align*}2x + 5 = 6 + 11\end{align*}@$
Subtraction Property of Equality If @$\begin{align*}a = b\end{align*}@$, then @$\begin{align*}a - c = b - c\end{align*}@$. If @$\begin{align*}m \angle x + 15^\circ = 65^\circ\end{align*}@$, then @$\begin{align*}m \angle x+15^\circ-15^\circ=65^\circ-15^\circ\end{align*}@$
Multiplication Property of Equality If @$\begin{align*}a = b\end{align*}@$, then @$\begin{align*}ac = bc\end{align*}@$. If @$\begin{align*}y = 8\end{align*}@$, then @$\begin{align*}5 \cdot y=5 \cdot 8\end{align*}@$
Division Property of Equality If @$\begin{align*}a = b\end{align*}@$, then @$\begin{align*}\frac{a}{c}=\frac{b}{c}\end{align*}@$. If @$\begin{align*}3b=18\end{align*}@$, then @$\begin{align*}\frac{3b}{3}=\frac{18}{3}\end{align*}@$
Distributive Property @$\begin{align*}a(b+c)=ab+ac\end{align*}@$ @$\begin{align*}5(2x-7)=5(2x)-5(7)=10x-35\end{align*}@$

Recall that @$\begin{align*}\overline{AB} \cong \overline{CD}\end{align*}@$ if and only if @$\begin{align*}AB = CD\end{align*}@$. @$\begin{align*}\overline{AB}\end{align*}@$ and @$\begin{align*}\overline{CD}\end{align*}@$ represent segments, while @$\begin{align*}AB\end{align*}@$ and @$\begin{align*}CD\end{align*}@$ are lengths of those segments, which means that @$\begin{align*}AB\end{align*}@$ and @$\begin{align*}CD\end{align*}@$ are numbers. The properties of equality apply to @$\begin{align*}AB\end{align*}@$ and @$\begin{align*}CD\end{align*}@$.

This also holds true for angles and their measures. @$\begin{align*}\angle ABC \cong \angle DEF\end{align*}@$ if and only if @$\begin{align*}m \angle ABC = m \angle DEF\end{align*}@$. Therefore, the properties of equality apply to @$\begin{align*}m \angle ABC\end{align*}@$ and @$\begin{align*}m \angle DEF\end{align*}@$.

Just like the properties of equality, there are properties of congruence. These properties hold for figures and shapes.

For Line Segments For Angles
Reflexive Property of Congruence @$\begin{align*}\overline{AB} \cong \overline{AB}\end{align*}@$ @$\begin{align*}\angle ABC \cong \angle CBA\end{align*}@$
Symmetric Property of Congruence If @$\begin{align*}\overline{AB} \cong \overline{CD}\end{align*}@$, then @$\begin{align*}\overline{CD} \cong \overline{AB}\end{align*}@$ If @$\begin{align*}\angle ABC \cong \angle DEF\end{align*}@$, then @$\begin{align*}\angle DEF \cong \angle ABC\end{align*}@$
Transitive Property of Congruence If @$\begin{align*}\overline{AB} \cong \overline{CD}\end{align*}@$ and @$\begin{align*}\overline{CD} \cong \overline{EF}\end{align*}@$, then @$\begin{align*}\overline{AB} \cong \overline{EF}\end{align*}@$ If @$\begin{align*}\angle ABC \cong \angle DEF\end{align*}@$ and @$\begin{align*}\angle DEF \cong \angle GHI\end{align*}@$, then @$\begin{align*}\angle ABC \cong \angle GHI\end{align*}@$

When you solve equations in algebra you use properties of equality. You might not write out the logical justification for each step in your solution, but you should know that there is an equality property that justifies that step. We will abbreviate “Property of Equality” “PoE” and “Property of Congruence” “PoC.”

Example A

Solve @$\begin{align*}2(3x-4)+11=x-27\end{align*}@$ and justify each step.

@$$\begin{align*}2(3x-4)+11 &= x-27\\ 6x-8+11 &= x-27 && \text{Distributive Property}\\ 6x+3 &= x-27 && \text{Combine like terms}\\ 6x+3-3 &= x-27-3 && \text{Subtraction PoE}\\ 6x &= x-30 && \text{Simplify}\\ 6x-x &= x-x-30 && \text{Subtraction PoE}\\ 5x &= -30 && \text{Simplify}\\ \frac{5x}{5} &= \frac{-30}{5} && \text{Division PoE}\\ x &= -6 && \text{Simplify}\end{align*}@$$

Example B

Given points @$\begin{align*}A, B\end{align*}@$, and @$\begin{align*}C\end{align*}@$, with @$\begin{align*}AB = 8, BC = 17\end{align*}@$, and @$\begin{align*}AC = 20\end{align*}@$. Are @$\begin{align*}A, B\end{align*}@$, and @$\begin{align*}C\end{align*}@$ collinear?

Set up an equation using the Segment Addition Postulate.

@$$\begin{align*}AB + BC &= AC && \text{Segment Addition Postulate}\\ 8 + 17 &= 20 && \text{Substitution PoE}\\ 25 & \neq 20 && \text{Combine like terms}\end{align*}@$$

Because the two sides are not equal, @$\begin{align*}A, B\end{align*}@$ and @$\begin{align*}C\end{align*}@$ are not collinear.

Example C

If @$\begin{align*}m \angle A+m \angle B=100^\circ\end{align*}@$ and @$\begin{align*}m \angle B = 40^\circ\end{align*}@$, prove that @$\begin{align*}\angle A\end{align*}@$ is an acute angle.

We will use a two-column format, with statements in one column and their corresponding reasons in the next. This is formally called a two-column proof.

Statement Reason
1. @$\begin{align*}m \angle A+m \angle B=100^\circ\end{align*}@$ and @$\begin{align*}m \angle B = 40^\circ\end{align*}@$ Given (always the reason for using facts that are told to us in the problem)
2. @$\begin{align*}m \angle A+40^\circ=100^\circ\end{align*}@$ Substitution PoE
3. @$\begin{align*}m \angle A = 60^\circ\end{align*}@$ Subtraction PoE
4. @$\begin{align*}\angle A\end{align*}@$ is an acute angle Definition of an acute angle, @$\begin{align*}m \angle A < 90^\circ\end{align*}@$

Watch this video for help with the Examples above.

CK-12 Foundation: Chapter2PropertiesofEqualityandCongruenceB

Guided Practice

Use the given property or properties of equality to fill in the blank. @$\begin{align*}x, y\end{align*}@$, and @$\begin{align*}z\end{align*}@$ are real numbers.

1. Symmetric: If @$\begin{align*}x = 3\end{align*}@$, then ______________.

2. Distributive: If @$\begin{align*}4(3x - 8)\end{align*}@$, then ______________.

3. Transitive: If @$\begin{align*}y = 12\end{align*}@$ and @$\begin{align*}x = y\end{align*}@$, then ______________.

Answers:

1. @$\begin{align*} 3=x\end{align*}@$

2. @$\begin{align*}12x-32\end{align*}@$

3. @$\begin{align*}x=12\end{align*}@$

Explore More

For questions 1-8, solve each equation and justify each step.

  1. @$\begin{align*}3x+11=-16\end{align*}@$
  2. @$\begin{align*}7x-3=3x-35\end{align*}@$
  3. @$\begin{align*}\frac{2}{3} g+1=19\end{align*}@$
  4. @$\begin{align*}\frac{1}{2} MN=5\end{align*}@$
  5. @$\begin{align*}5m \angle ABC=540^\circ\end{align*}@$
  6. @$\begin{align*}10b-2(b+3)=5b\end{align*}@$
  7. @$\begin{align*}\frac{1}{4}y+\frac{5}{6}=\frac{1}{3}\end{align*}@$
  8. @$\begin{align*}\frac{1}{4}AB+\frac{1}{3}AB=12+\frac{1}{2}AB\end{align*}@$

For questions 9-12, use the given property or properties of equality to fill in the blank. @$\begin{align*}x, y\end{align*}@$, and @$\begin{align*}z\end{align*}@$ are real numbers.

  1. Symmetric: If @$\begin{align*}x + y = y + z\end{align*}@$, then _________.
  2. Transitive: If @$\begin{align*}AB = 5\end{align*}@$ and @$\begin{align*}AB = CD\end{align*}@$, then _________.
  3. Substitution: If @$\begin{align*}x = y - 7\end{align*}@$ and @$\begin{align*}x = z + 4\end{align*}@$, then _________.
  4. Distributive: If @$\begin{align*}3(2x-4)=y\end{align*}@$, then_____.
  5. Given points @$\begin{align*}E, F\end{align*}@$, and @$\begin{align*}G\end{align*}@$ and @$\begin{align*}EF = 16\end{align*}@$, @$\begin{align*}FG = 7\end{align*}@$ and @$\begin{align*}EG = 23\end{align*}@$. Determine if @$\begin{align*}E, F\end{align*}@$ and @$\begin{align*}G\end{align*}@$ are collinear.
  6. Given points @$\begin{align*}H, I\end{align*}@$ and @$\begin{align*}J\end{align*}@$ and @$\begin{align*}HI = 9, IJ = 9\end{align*}@$ and @$\begin{align*}HJ = 16\end{align*}@$. Are the three points collinear? Is @$\begin{align*}I\end{align*}@$ the midpoint?
  7. If @$\begin{align*}m \angle KLM = 56^\circ\end{align*}@$ and @$\begin{align*}m \angle KLM + m \angle NOP = 180^\circ\end{align*}@$, explain how @$\begin{align*}\angle NOP\end{align*}@$ must be an obtuse angle.

Vocabulary

Real Number

Real Number

A real number is a number that can be plotted on a number line. Real numbers include all rational and irrational numbers.
Right Angle Theorem

Right Angle Theorem

The Right Angle Theorem states that if two angles are right angles, then the angles are congruent.
Same Angle Supplements Theorem

Same Angle Supplements Theorem

The Same Angle Supplements Theorem states that if two angles are supplementary to the same angle then the two angles are congruent.
Vertical Angles Theorem

Vertical Angles Theorem

The Vertical Angles Theorem states that if two angles are vertical, then they are congruent.

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