Variables and Expressions
Algebra involves the solution of problems using variables, expressions, and equations. This topic focuses on variables and expressions and you will learn about the types of expressions used in algebra.
One thing that separates algebra from arithmetic is the variableA letter or symbol used to represent a quantity that can change. . A variable is a letter or symbol used to represent a quantity that can change. Any letter can be used, but `x` and `y` are common. You may have seen variables used in formulas, like the area of a rectangle. To find the area of a rectangle, you multiply length times width, written using the two variables `l` and `w`.
`l*w`
Here, the variable `l` represents the length of the rectangle. The variable `w` represents the width of the rectangle.
You may be familiar with the formula for the area of a triangle. It is `1/2b*h`.
Here, the variable `b` represents the base of the triangle, and the variable `h` represents the height of the triangle. The `1/2` in this formula is a constantA symbol that represents a quantity that cannot change. It can be a number, letter or a symbol.. A constant, unlike a variable, is a quantity that does not change. A constant is often a number.
An expressionA mathematical phrase that can contain a combination of numbers, variables, or operations. is a mathematical phrase made up of a sequence of mathematical symbols. Those symbols can be numbers, variables, or operations `(+, -, *, -:)`. Examples of expressions are `l*w` and `1/2b*h`.
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Example |
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Problem
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Identify the constant and variable in the expression `24 - x`. |
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Answer |
`24` is the constant. `x` is the variable. |
Since `24` cannot change its value, it is a constant. The variable is `x`, because it could be `0`, or `2`, or many other numbers. |
In arithmetic, you often evaluated, or simplified, expressions involving numbers.
`3*25+4` `25-:5` `142-12/4` `3/4-1/4` `2.45 + 13`
In algebra, you will evaluate many expressions that contain variables.
`a+10` `48*c` `100-x` `l*w` `1/2b*h`
To evaluateTo find the value of an expression. an expression means to find its value. If there are variables in the expression, you will be asked to evaluate the expression for a specified value for the variable.
The first step in evaluating an expression is to substituteThe replacement of a variable with a number. the given value of a variable into the expression. Then you can finish evaluating the expression using arithmetic.
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Example |
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Problem |
Evaluate `24 - x` when `x = 3`. |
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`24 - x` `24 - 3` |
Substitute `3` for the `x` in the expression.
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`24 - 3 = 21`
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Subtract to complete the evaluation. |
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`21` |
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When you have two variables, you substitute each given value for each variable.
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Example |
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Problem |
Evaluate `l*w` when `l=3` and `w=8`. |
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`l*w` `3*8` |
Substitute `3` for `l` in the expression and `8` for `w`. |
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`3*8=24` |
Multiply. |
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`24` |
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When you multiply a variable by a constant number, you don’t need to write the multiplication sign or use parentheses. For example, `3a` is the same as `3*a`.
Notice that the sign `*` is used to represent multiplication. This is because the multiplication sign x looks a lot like the letter `x`, especially when hand written. Because of this, it’s best to use parentheses or the `*` sign to indicate multiplication of numbers.
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Example |
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Problem |
Evaluate `4x - 4` when `x = 10`. |
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`4x - 4`
`4(10) - 4` |
Substitute `10` for `x` in the expression.
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`40 - 4` `36` |
Remember that you must multiply before you do the subtraction. |
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Answer |
`36` |
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Since the variables are allowed to vary, there are times when you want to evaluate the same expression for different values for the variable.
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Example |
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Problem
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John is planning a rectangular garden that is `2` feet wide. He hasn’t decided how long to make it, but he’s considering `4` feet, `5` feet, and `6` feet. He wants to put a short fence around the garden. Using `x` to represent the length of the rectangular garden, he will need `x + x + 2 + 2`, or `2x + 4`, feet of fencing.
How much fencing will he need for each possible garden length? Evaluate the expression when `x=4,` `x = 5`, and `x = 6` to find out. |
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`2x + 4` `2(4) + 4` `8 + 4` `12` |
For `x = 4`, substitute `4` for `x` in the expression. Evaluate by multiplying and adding. |
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`2x + 4` `2(5) + 4` `10 + 4` `14` |
For `x = 5`, substitute `5` for `x`. Evaluate by multiplying and adding. |
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`2x + 4` `2(6) + 4` `12 + 4` `16` |
For `x = 6`, substitute `6` for `x` and evaluate.
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Answer |
John needs `12` feet of fencing when `x=4,` `14` feet when `x=5,` and `16` feet when `x=6`. |
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Evaluating expressions for many different values for the variable is one of the powers of algebra. Computer programs are written to evaluate the same expression (usually a very complicated expression) for millions of different values for the variable(s).
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Evaluate `8x - 1` when `x = 2`.
A) `1`
B) `7`
C) `8`
D) `15`
E) `16`
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Variables are an important part of algebra. Expressions made from variables, constants, and operations can represent a numerical value. You can evaluate an expression when you are provided with one or more values for the variables: substitute each variable’s value for the variable, then perform any necessary arithmetic.