Evaluating Expressions

You have already learned how to represent unknown quantities with variables. Last week, you learned how to find the value of expressions containing variables when given a value of the variable. Today, you will continue to find the value of expressions containing variables when given a value of the variable.

 Evaluate means to find the value of an expression.  To evaluate an algebraic expression, use substitution to replace the variable with a number.

How can you evaluate an algebraic expression?

Willie has one large case that holds 20 miniature racecars. He also has 3 smaller cases with miniature racecars. The number of miniature cars Willie has can be expressed as. How many miniature cars does he have if each smaller case holds 14 cars?

 If x equals the number of miniature cars in each smaller case, then evaluate for x =14. Evaluate Correct Algebraic Expression

Can you use the distributive property and mental math? Willie has 62 miniature racecars.

 Why did you substitute 14 for x ? [x stands for the number of cars in each small case and the problem ask how many cars there would be if each small case holds 14 cars]  Why do you multiply 3 by 14 before adding? [The order of operations says to multiply first.]

 Suppose that the smaller cases each hold 10 miniature cars. How many cars would he have then? Evaluate for x = 10

Can you use the distributive property and mental math? Willie would have 50 miniature racecars.

x20+3x What does the first row in the table show? [It shows the relationship: if there are x cars in a small case, then the total number of cars is 20+3x.] What do the next two rows in the table show? [The middle row shows how many cars Willie would have if there were 14 cars in each small case and the next row shows the number of cars Willie would have if there were 10 cars in each small case.]