PRE-ALGEBRA: Evaluating Expressions

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Objective

SWBAT evaluate an algebraic expression by substituting in one or more values for variables. SWBAT show that they understand order of operations when evaluating expressions.

Big Idea

Students learn best when they can build their own meaning around content. In this lesson, the students will have a chance to play with numbers while using order of operations to evaluate expressions.

Do Now

5 minutes

Slide 3: Students can determine if the expressions have the same values individually.  Then have them share their results with their partner.  I will call on one pair to share out about why the two expressions have the same value (this expression was chosen for two reasons 1) students often mistake 8^2 as sixteen and 2) students often forget that a negative number squared must equal a positive value.  This pair of expressions aims to build some understanding around these two points).  Once one group has shared, I like to look to one or two other pairs to build on what the initial group has said.

Opening

5 minutes

Slide 4: Have students evaluate the expression on their own without discussing it with their partners.  I then call on students to give the answer(s) and I record them on the board.  Typically, there will be several answers from the class due to the fact that students are not using the order of operations properly.  After making a list of all the answers that the class gives (I do not give any indication as to which one is correct) I ask the class if they see a problem with what just happened?  I ask them to think about this for 30 seconds on their own and then I have them share their thoughts with their partner as I walk around to hear what the partnerships are thinking.  When we share out with the whole class usually a student will say that “we got different answers.” 

This is a start, but we want to guide the conversation towards the understanding that we cannot all simplify the same expression and get multiple answers.  I explain to students that arithmetic is guided by a set of rules that indicate that things need to be done in a certain order.

Slide 5: Students have all seen this many times and I just put it up as a reminder and then we go back to slide 3.  I give students a second opportunity to simplify the expression again with their partner.

When students were sharing the answers, I made note of the groups that arrived at the correct solution of 23.  I let one of these groups share from their seat or come up to the board to explain the proper order in which to simplify the expression. 

Investigation

15 minutes

Direction Instruction + Guided Practice

10 minutes

Slide 7: While substituting and evaluating expressions should not be new for students, it is important to review this skill as it will help to lay the foundation for evaluating functions in this unit.  I put these exercises on the board and let students work on them in their partnerships without any instruction.  Students will then have the opportunity to put their answers on the board or under the document camera to explain their work and ensure that the remainder of the class is in agreement of how to simplify the expressions.

Closure

10 minutes

Slide 8: In this ticket out students are working with their partner to discuss an error analysis question where a student made a mistake.  Students should work with their partner to write what the mistake was and how to fix it, showing the correct solution as a results.

 

Slide 9: This final question requires students to critique the reasoning of another student (MP3) and explain why they are correct or incorrect.  As a scaffold, if students are having some difficulty with this question I will encourage them to evaluate both expressions first in order to determine if they are equivalent or not.  Since this lesson takes place in the beginning of the year,  I take some extra time to show what a good explanation might look like for this question using student work.  The most important thing is that students are citing mathematics directly from the problem to make their case.

  e.g. Noella is incorrect in her thinking.  While she is correct that a fraction line does mean division, when a fraction is written vertically as in the first expression it is implied to evaluate the top (10 times 15) and bottom (5 times 3) of the fraction first.  When the expression is written horizontally as in the second expression, order of operations would dictate that we do 15 divided by 5 first.