# Algebra/Equations

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Algebra
 ← Sets Equations Solving Equations →
 To do: examples, practice quizzes and review quizzes (as applicable)

## Objectives

• Define expressions.
• State the definition of a variable and an expression.
• State the definition of a constant.
• Explain the difference between an expression and an equation.
• Given a written description of a situation, write expressions for the unknowns, and then relate those expressions with equations.

## Balancing variables

Mathematics began with the concept of counting, but it grew because of the idea of equivalence. Tally marks or counters are equivalent to a value depending on the concept being expressed. For instance a shepherd could use a bag of rocks to make sure that he brought home the same number of sheep he left with. The idea of equivalence as shown with an equal sign allows us to state "is the same as".

An expression is a statement that is well formed. The neolithic shepherd used his bag of rocks to create expressions for managing his herd. For instance, the shepherd hoped that the size of his flock remained constant between the time he took his flock out to graze and the time that he returned them home. When he used the bag of rocks the shepherd used two expressions: # Sheep and # Rocks to create the equation # Sheep = # Rocks. Mathematics describes the operations the shepherd performed on the expressions in order to keep the equation in balance. For instance, when the shepherd used his bag of rocks to count his flock at night he might put his rocks in a bowl next to the gate on his pen. Each time a sheep went into the pen the shepherd would move a rock from the bag into the bowl. If rocks were left over in the bag, then the shepherd knew he needed to return to the field to find his lost sheep.

## Expressions

An expression is the arrangement of mathematical symbols denoting values and operations, while the equation is a form of an expression that indicates that two values are equal. While equations are generally expressions, you will discover that some expressions are not equations. Expressions in the form of functions or inequalities will be more apparent later in the book.

When you simplify an expression, you are performing simple arithmetic steps until you receive the simplest answer, which is usually an integer or fraction.

## Equations

A balance (scale).

An equation is just like a balance. A balance is a machine that compares whether two quantities are the same or not the same. In mathematics, two quantities balanced translates to "having the same value." An equation is true if the two sides are balanced and false if the two sides are not balanced. As 5=5. This is a very simple equation. We can use variables too(almost used in mathematics)e.g. x=5. This is same as while using a balancing machine, we use some quantity on one side and some matter on the other which is to be measured. As an example we know that 5 is equal to 5 and x is equal to x(whatever its value is). The simplest example of a false equation is that 5 is not equal to 20.

• Add examples.

Remember from the previous section, a variable is a symbol, usually a letter, that stands for a number. We'll use the letters ${\displaystyle x}$ and ${\displaystyle y}$ most often. If ${\displaystyle x=5}$ is true, then any time I use ${\displaystyle x}$, you can replace ${\displaystyle x}$ by 5. We're saying ${\displaystyle x}$ and 5 have the same value, but different appearance. In the equation ${\displaystyle x=5}$, the equal sign is like the balance. If ${\displaystyle x=5}$ is true, we know that whenever we see an ${\displaystyle x}$ we can substitute the number 5 if that helps us understand. Even more important, we can check if a given number substituted for a variable in an equation makes that equation true.

• Add examples.

Frequently we don't know what the value of a variable is that would make the equation true, and we need to find out! Finding out the values that make an equation true is called Solving an Equation, which we will do without guessing in the next section.

• Add examples.

## Lesson Quiz

You might be tempted to rush ahead, but please quickly check your understanding first!

1. Evaluate the expression ${\displaystyle ab^{2}\cdot \!(ac)^{2}+c(b-a)}$ if ${\displaystyle a=2}$, ${\displaystyle b=(-3)}$, and ${\displaystyle c=(-2)}$.
2. I put $300 into a savings account that has an Interest Rate of 4.5% per year, and I've had this account for exactly a year now. How much do I have in total (Principal with Interest Earned added on)? 3. To find the area of a trapezoid, we use this formula: ${\displaystyle A={\frac {1}{2}}h(b_{1}+b_{2})}$, where ${\displaystyle h}$ represents the perpendicular height of the trapezoid, ${\displaystyle b_{1}}$ represents one of the bases, and ${\displaystyle b_{2}}$ represents the other base. What would the area of a trapezoid be with ${\displaystyle h=4}$, ${\displaystyle b_{1}=3}$, and ${\displaystyle b_{2}=13}$? 4. ${\displaystyle F={\frac {9}{5}}C+32}$ is the formula to convert Fahrenheit to Celsius or vice versa. What would the temperature in Fahrenheit if it was 25 Degrees Celsius? 5. ${\displaystyle C=2\pi \!r}$ is the formula to find the Circumference of a circle with a given radius. Find the Circumference if the diameter was 4 inches long. Remember: 2r = d 6. ${\displaystyle I=PRT\,}$ when I represents interest earned, P represents the principal (starting money), R represents interest rate, and T represents the time, usually express in years. This formula is often used for banking and account. After 2 years, how much interest would have been built up if you placed$5000 into an account that has an interest rate of 9.5% per year?
7. Does ${\displaystyle a^{(b^{c})}=(a^{b})^{c}}$?

### Quiz Answers

1. 2980
2. \$313.5
3. 32
4. ${\displaystyle {\begin{matrix}F&=&\left({\frac {9}{5}}\right)(25)+32\\\ &=&45+32\\\ &=&77\end{matrix}}}$
5. Assuming that ${\displaystyle \pi }$ = 3.14:
${\displaystyle {\begin{matrix}C&=&2\pi \!r\quad AND\quad d&=&2r\\\ &=&4\pi \\\ &\approx &12.56\end{matrix}}}$
6. ${\displaystyle {\begin{matrix}I&=&(5000)(.095)(2)\\\ &=&950\end{matrix}}}$
7. No