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Chapter 1: Variables and Expressions
 
A comet raced through our solar system traveling at approximately 3.8 million miles per day. The table shows how far the comet will travel in 1 day, 2 days and 3 days. An expression can be written to describe how far the comet will travel in any number of days.

 Days

Expression

Distance Traveled (in miles)

 1

3,800,000 x 1

3,800,000

 2

 3,800,000 x 2

7,600,000

 3

 3,800,000 x 3

11,400,000

 d

3,800,000 x d
  
Symbols are used in algebra to represent numbers. When a symbol represents a number it is called a variable. Any letter can be used to represent on or more numbers.

In the previous example, the variable (d) represents the number of days that the comet travels.

 3,800,000 x d
The expression shown above can be used to find the distance the comet travels in any number of days.
 

Let's evaluate an expression by substituting a value for the variable.

Evaluate for x=8.

 
 
Substituting for x, where x = 8.

 x+4

 8 + 4

 12
Evaluate these expressions:

 a + 6 for a = 13

 22+c-9 for c=18

b + 12 for b = 19

 18-d for d = 5

 
Evaluate these expressions:

 15x for x = 6

 18/3y for y=2

x/15 for x = 45

 3a-2 for a=9

 
Evaluate these expressions:

 (a+b)/3 for a=9, b=12

 (3+y)/x for y=13,x=4
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