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One-Step Equations and Inverse Operations

Add and subtract like terms as a step to solve equations.

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Solving One Step Equations

San Francisco

Credit: Aleksandar Jovanovic
Source: http://www.freeimages.com/photo/san-francisco-1451843
License: CC BY-NC 3.0

Alex lives in Los Angeles and his best friend Gabriel lives 380 miles away in San Francisco. If he drives at a constant speed of 65 miles per hour all the way there, how long will it take Alex to get to San Francisco? (Assuming he doesn't stop.)

Linear Equations

When solving an equation for a variable, you must get the variable by itself. All the equations in this lesson are linear equations. That means the equation can be simplified to \begin{align*}ax+b=c\end{align*}, where \begin{align*}a, b\end{align*}, and \begin{align*}c\end{align*} are real numbers. Here we will only deal with using one operation; addition, subtraction, multiplication or division.

Solve each equation

Solve \begin{align*}7+y=16\end{align*} for \begin{align*}y\end{align*}.

This problem is simple and you could probably solve it in your head. However, to start good practices, you should always use algebra to solve any equation. Even if the problem seems easy, equations will get more difficult to solve.

To solve an equation for a variable, you must do the opposite, or undo, whatever is on the same side as the variable. 7 is being added to \begin{align*}y\end{align*}, so we must subtract 7 from both sides. Notice that this is very similar to the previous concept (Solving Algebraic Equations for a Variable).

\begin{align*}& \ \bcancel{7}+y = 16\\ & \underline{-\bcancel{7} \quad \quad -7 \; \;}\\ & \ \quad \ \ y = 9\end{align*}

You can check that \begin{align*}y= 9\end{align*} is correct by plugging 9 back into the original equation. 7 + 9 does equal 16, so we know that we found the correct answer.

Solve \begin{align*}-7h=84\end{align*}.

Recall that \begin{align*}-7h = -7 \times h\end{align*}, so the opposite, or inverse, operation of multiplication is division. Therefore, we must divide both sides by -7 to solve for \begin{align*}h\end{align*}.

\begin{align*}\frac{-\bcancel{7}h}{-\bcancel{7}} &= \frac{84}{-7}\\ h &= -12\end{align*}

Again, check your work. \begin{align*}-7 \cdot -12\end{align*} is equal to 84, so we know our answer is correct.

Solve \begin{align*}\frac{3}{8} x = \frac{3}{2}\end{align*}.

The variable is being multiplied by a fraction. Instead of dividing by a fraction, we multiply by the reciprocal of \begin{align*}\frac{3}{8}\end{align*}, which is \begin{align*}\frac{8}{3}\end{align*}.

\begin{align*}\xcancel{\frac{8}{3} \cdot \frac{3}{8}} x &= \bcancel{\frac{3}{2} \cdot \frac{8}{3}}\\ x &= \frac{8}{2}=4\end{align*}

Check the answer: \begin{align*}\frac{3}{_2\cancel{8}} \cdot \cancel{4}=\frac{3}{2}\end{align*}. This is correct, so we know that \begin{align*}x = 4\end{align*} is the answer.

Examples

Example 1

Earlier, you were asked how long will it take Alex to get to San Francisco.

Set up an equation to represent Alex's travel, \begin{align*}t=d \div 65\end{align*} where t is time and d is distance. Therefore, it takes him \begin{align*}t=380 \div 65\end{align*} or 5.85 hours to get to Gabriel's house, which is 5 hours and 51 minutes.

Solve the following equations for the given variable. Check your answer.

Example 2

\begin{align*}5+j=17\end{align*}

Subtract 5 from both sides to solve for \begin{align*}j\end{align*}.

\begin{align*}& \ \ \bcancel{5}+j=17\\ & \underline{-\bcancel{5} \qquad -5 \; \;}\\ & \qquad \ j=12\end{align*}

Check the answer: \begin{align*}5+12=17 \end{align*}

Example 3

\begin{align*}\frac{h}{6}=-11\end{align*}

\begin{align*}h\end{align*} is being divided by 6. To undo division, we must multiply both sides by 6.

\begin{align*}\cancel{6} \cdot \frac{h}{\cancel{6}} &= -11.6\\ h &= -66\end{align*}

Check the answer: \begin{align*}\frac{-66}{6}=-11 \end{align*}

Example 4

\begin{align*}\frac{5}{4}x=35\end{align*}

Multiply both sides by the reciprocal of \begin{align*}\frac{5}{4}\end{align*}

\begin{align*}\frac{\cancel{4}}{\cancel{5}} \cdot \frac{\cancel{5}}{\cancel{4}}x &= _7\cancel{35} \cdot \frac{4}{\cancel{5}}\\ x &=28\end{align*}

Check the answer: \begin{align*}\frac{5}{4} \cdot 28 = 5 \cdot 7 = 35\end{align*}

Review

Solve each equation below and check your answer. Reduce any fractions.

  1. \begin{align*}-3+x=-1\end{align*}
  2. \begin{align*}r+6=2\end{align*}
  3. \begin{align*}5s=30\end{align*}
  4. \begin{align*}-8k=-64\end{align*}
  5. \begin{align*}\frac{m}{-4}=14\end{align*}
  6. \begin{align*}90=10n\end{align*}
  7. \begin{align*}-16=y-5\end{align*}
  8. \begin{align*}\frac{6}{7}d=36\end{align*}
  9. \begin{align*}6=-\frac{1}{3}p\end{align*}
  10. \begin{align*}u-\frac{3}{4}=\frac{5}{6}\end{align*}
  11. \begin{align*}\frac{8}{5}a=-\frac{72}{13}\end{align*}
  12. \begin{align*}\frac{7}{8}=b+\frac{1}{2}\end{align*}
  13. \begin{align*}w-(-5)=16\end{align*}
  14. \begin{align*}\frac{1}{4}=b-\left(-\frac{2}{5}\right)\end{align*}
  15. \begin{align*}\frac{3}{5}q=-\frac{12}{11}\end{align*}
  16. \begin{align*}\frac{t}{12}=-4\end{align*}
  17. \begin{align*}45=15x\end{align*}
  18. \begin{align*}7=\frac{g}{-8}\end{align*}

Challenge Solve the equation below. Be careful!

  1. \begin{align*}14-z=-3\end{align*}
  2. Alex decides he'd rather drive 70 miles an hour to get up to San Francisco. How long will it take him to drive up north now? Remember it was 380 miles to get to San Francisco from Los Angeles.

Answers for Review Problems

To see the Review answers, open this PDF file and look for section 1.7. 

Vocabulary

constant

A constant is a value that does not change. In Algebra, this is a number such as 3, 12, 342, etc., as opposed to a variable such as x, y or a.

Equation

An equation is a mathematical sentence that describes two equal quantities. Equations contain equals signs.

Inverse

Inverse operations are operations that "undo" each other. Multiplication is the inverse operation of division. Addition is the inverse operation of subtraction.

linear equation

A linear equation is an equation between two variables that produces a straight line when graphed.

Numerical Coefficient

In mathematical expressions, the numerical coefficients are the numbers associated with the variables. For example, in the expression 4x, 4 is the numerical coefficient and x is the variable.

reciprocal

The reciprocal of a number is the number you can multiply it by to get one. The reciprocal of 2 is 1/2. It is also called the multiplicative inverse, or just inverse.

Variable

A variable is a symbol used to represent an unknown or changing quantity. The most common variables are a, b, x, y, m, and n.

Image Attributions

  1. [1]^ Credit: Aleksandar Jovanovic; Source: http://www.freeimages.com/photo/san-francisco-1451843; License: CC BY-NC 3.0

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