2.7.b – Solving Equations Containing Fractions Using the Multiplication Property of Equality

Learning Outcomes

Solve equations with fractions using the Multiplication Property of Equality

Solve Equations with Fractions Using the Multiplication Property of Equality

We will now solve equations that require multiplication to isolate the variable. Consider the equation [latex]\Large\frac{x}{4}\normalsize=3[/latex]. We want to know what number divided by [latex]4[/latex] gives [latex]3[/latex]. To “undo” the division, we will need to multiply by [latex]4[/latex]. The Multiplication Property of Equality will allow us to do this. This property says that if we start with two equal quantities and multiply both by the same number, the results are equal.

The Multiplication Property of Equality

For any numbers [latex]a,b[/latex], and [latex]c[/latex],

[latex]\text{if }a=b,\text{ then }ac=bc[/latex].

If you multiply both sides of an equation by the same quantity, you still have equality.

Let’s use the Multiplication Property of Equality to solve the equation [latex]\Large\frac{x}{7}\normalsize=-9[/latex].

Example

Solve: [latex]\Large\frac{x}{7}\normalsize=-9[/latex].

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Example

Solve: [latex]\Large\frac{p}{-8}\normalsize=-40[/latex]

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In the following video we show two more examples of when to use the multiplication and division properties to solve a one-step equation.

Solve Equations with a Coefficient of [latex]-1[/latex]

Look at the equation [latex]-y=15[/latex]. Does it look as if [latex]y[/latex] is already isolated? But there is a negative sign in front of [latex]y[/latex], so it is not isolated.

There are three different ways to isolate the variable in this type of equation. We will show all three ways in the next example.

Example

Solve: [latex]-y=15[/latex]

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In the next video we show more examples of how to solve an equation with a negative variable.

Solve Equations with a Fraction Coefficient

When we have an equation with a fraction coefficient we can use the Multiplication Property of Equality to make the coefficient equal to [latex]1[/latex].

For example, in the equation:

[latex]\Large\frac{3}{4}\normalsize x=24[/latex]

The coefficient of [latex]x[/latex] is [latex]\Large\frac{3}{4}[/latex]. To solve for [latex]x[/latex], we need its coefficient to be [latex]1[/latex]. Since the product of a number and its reciprocal is [latex]1[/latex], our strategy here will be to isolate [latex]x[/latex] by multiplying by the reciprocal of [latex]\Large\frac{3}{4}[/latex]. We will do this in the next example.

Example

Solve: [latex]\Large\frac{3}{4}\normalsize x=24[/latex]

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Example

Solve: [latex]-\Large\frac{3}{8}\normalsize w=72[/latex]

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In the next video example you will see another example of how to use the reciprocal of a fractional coefficient to solve an equation.

		[latex]\Large\frac{x}{7}\normalsize=-9[/latex]
Use the Multiplication Property of Equality to multiply both sides by [latex]7[/latex] . This will isolate the variable.		[latex]\color{red}{7}\cdot\Large\frac{x}{7}\normalsize=\color{red}{7}(-9)[/latex]
Multiply.		[latex]\Large\frac{7x}{7}\normalsize=-63[/latex]
Simplify.		[latex]x=-63[/latex]
Check. Substitute [latex]\color{red}{-63}[/latex] for x in the original equation.	[latex]\Large\frac{\color{red}{-63}}{7}\normalsize\stackrel{?}{=}-9[/latex]
The equation is true.	[latex]-9=-9\quad\checkmark[/latex]

		[latex]\Large\frac{p}{-8}\normalsize=-40[/latex]
Multiply both sides by [latex]-8[/latex]		[latex]\color{red}{-8}\Large(\frac{p}{-8})\normalsize=\color{red}{-8}(-40)[/latex]
Multiply.		[latex]\Large\frac{-8p}{-8}\normalsize=320[/latex]
Simplify.		[latex]p=320[/latex]
Check:
Substitute [latex]p=320[/latex] .	[latex]\Large\frac{\color{red}{320}}{-8}\normalsize\stackrel{?}{=}-40[/latex]
The equation is true.	[latex]-40=-40\quad\checkmark[/latex]

	[latex]-y=15[/latex]
Rewrite [latex]-y[/latex] as [latex]-1y[/latex] .	[latex]-1y = 15[/latex]
Divide both sides by [latex]−1[/latex].	[latex]\Large\frac{-1y}{\color{red}{-1}}=\Large\frac{15}{\color{red}{-1}}[/latex]
Simplify each side.	[latex]y = -15[/latex]

	[latex]y=15[/latex]
Multiply both sides by [latex]−1[/latex].	[latex]\color{red}{-1}(-y)=\color{red}{-1}(15)[/latex]
Simplify each side.	[latex]y=-15[/latex]

	[latex]y=15[/latex]
Substitute [latex]y=-15[/latex] .	[latex]-(\color{red}{-15})\stackrel{?}{=}(15)[/latex]
Simplify. The equation is true.	[latex]15=15\quad\checkmark[/latex]

Module 2: Fractions