Writing Addition and Subtraction Expressions
Expressions can be modeled using a bar diagram like the one below. This example shows the number of biographies (x) currently in our classroom library and the increase of 13 new biographies from the school book fair. This expression x + 13 represents the total number of biographies the library has now.
This expression can also be written as 13 + x to represent this situation. One way to test the equivalency of the expressions is to substitute values for x and see if the sum of both equations are still the same. If x = 2, x+ 13 = 13+ x, 2 + 13 = 13 + 2, If x= -1 -1 + 13 = 13 + -1
As a result of the commutative property of addition these expressions are also proven equivalent.
Write my own algebraic expression...
1. Start with an input such as "12" (miles from home to downtown)
2. Add in a variable such as "d" (distance traveled)
3. Put it together into a situation like I want to get downtown but have already traveled d miles. How far do I still have to go?
4. Define your variable(s) D= distance traveled.
5. Finally, Write the expression 12 - d
Now test out your algebraic expression.... If d= 6.5 how far is the distance from downtown?
12- d Substitute in 6.5 for d, 12-6.5= 5
If d= 6.5, I have to go only 5 more miles.
Sample Practice Questions about writing expressions. Write an algebraic expression to represent each of the following situations.
- The height (h) increased by 6 inches.
- 43 more than (t); t= time
- Carrie sold 50 bags of popcorn today. Carrie sold (p) fewer bags than Terry.
- Seven less than a number (n)
