Lesson 8
Sumemos y restemos fracciones
Warm-up: Cuál es diferente: Representaciones de fracciones (10 minutes)
Narrative
The purpose of this Which One Doesn’t Belong is for students to recall representations of fractions they have seen in an earlier course. Two of the representations are fraction strips and the other two are number lines. These representations will be useful to students in this and future lessons as they think about representing equivalent fractions.
Launch
- Groups of 2
- Display the image.
- “Escojan una que sea diferente. Prepárense para explicar por qué es diferente” // “Pick one that doesn’t belong. Be ready to share why it doesn’t belong.”
- 1 minute: quiet think time
Activity
- “Discutan con su compañero cómo pensaron” // “Discuss your thinking with your partner.”
- 2–3 minutes: partner discussion
- Share and record responses.
Student Facing
¿Cuál es diferente?
Student Response
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Activity Synthesis
- “¿Cómo nos ayudan los diagramas B y C a entender la relación que hay entre tercios, sextos y doceavos?” // “How do the diagrams in B and C help us see the relationship between thirds, sixths, and twelfths?” (We can see that \(\frac{1}{3}=\frac{4}{12}\) and \(\frac{1}{3}=\frac{2}{6}\).)
Activity 1: Clasificación de tarjetas: Sumas y diferencias de fracciones (15 minutes)
Narrative
The purpose of this activity is for students to sort expressions showing sums and differences of fractions. The cards include mixed numbers and expressions with the same denominator or with different denominators. Students are not expected to find the value of the expressions as that will be the work of the next activity. One way of sorting, however, may be based on whether or not they know how to find the value of the expression.
Advances: Speaking, Conversing
Required Materials
Materials to Copy
- Fraction Add and Subtract Sort
Required Preparation
- Create a set of cards from the blackline master for each group of 2.
Launch
- Groups of 2 or 4
- Distribute one set of pre-cut cards to each group of students.
- “En esta actividad, van a clasificar algunas tarjetas en las categorías que quieran. Cuando clasifiquen las expresiones, invéntense las categorías con su compañero” // “In this activity, you will sort some cards into categories of your choosing. When you sort the expressions, you should work with your partner to come up with categories.”
Activity
- 8 minutes: partner work time
- Monitor for students who sort the expressions according to whether the denominators of the fractions are the same or different.
Student Facing
Tu profesor te va a dar un grupo de tarjetas que muestran expresiones.
- Clasifica las tarjetas en 2 categorías, las que quieras. Prepárate para explicar el significado de tus categorías.
- Clasifica las tarjetas en otras 2 categorías que sean diferentes a las anteriores. Prepárate para explicar el significado de tus nuevas categorías.
Student Response
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Activity Synthesis
- Select groups to share their categories and how they sorted their cards.
- Choose as many different types of categories as time allows, but ensure that one set of categories distinguishes between expressions that have the same denominator and expressions that have different denominators.
- Display: \(\frac{2}{3} - \frac{1}{3}\)
- “¿Cómo pueden encontrar el valor de esta expresión?” // “How could you find the value of this expression?” (I can just take 1 from 2 since they are both thirds.)
- Display: \(\frac{2}{3} - \frac{1}{6}\)
- “¿Por qué la forma de encontrar el valor de esta expresión es diferente?” // “Why is finding the value of this expression different?” (It’s thirds and sixths so I can’t just take the sixth away.)
- “En la siguiente actividad vamos a encontrar los valores de expresiones como estas” // “In the next activity we will find the values of expressions like these.”
Activity 2: Sumemos y restemos (20 minutes)
Narrative
The purpose of this activity is for students to add and subtract fractions in a way that makes sense to them. Students may use strategies such as drawing tape diagrams or number lines, or they may use computations to find a common denominator. Monitor for and select students with the following strategies to share in the synthesis:
- use the meaning of fractions to explain why \(\frac{2}{3}+\frac{2}{3}=\frac{4}{3}\)
- use a diagram like a number line to find the value of \(\frac{2}{3} - \frac{1}{6}\) and \(\frac{2}{3}+\frac{1}{2}\)
- use equivalent fractions and arithmetic to find the value of \(\frac{2}{3} - \frac{1}{6}\) and \(\frac{2}{3}+\frac{1}{2}\)
Students who choose to use the number line or tape diagrams use appropriate tools strategically (MP5).
Supports accessibility for: Organization, Conceptual Processing, Language
Launch
- Groups of 4
Activity
- 5 minutes: independent work time
- 5 minutes: small-group discussion
- As students work, consider asking:
- “¿En qué se parecen estas expresiones? ¿En qué son diferentes?” // “What is the same about these expressions? What is different?”
- “¿Cómo decidieron cuál estrategia usar?” // “How did you decide which strategy to use?”
Student Facing
Encuentra el valor de cada expresión. Explica o muestra cómo pensaste.
- \(\frac{2}{3}+\frac{2}{3}\)
- \(\frac{2}{3}-\frac{1}{6}\)
- \(\frac{2}{3}+\frac{1}{2}\)
Student Response
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Advancing Student Thinking
If students do not have a strategy to add or subtract fractions with unlike denominators, refer to one of the expressions and ask, “¿Cómo puedes usar una recta numérica para mostrar estas dos fracciones?” // “How could you use one number line to show both of these fractions?”
Activity Synthesis
- Ask selected students to share their response for how to add or subtract fractions with the same denominator.
- “¿En qué fue diferente esta suma a las otras 2 sumas?” // “How was this sum different than the other 2 sums?” (It was thirds and thirds so I could just add them. I did not need to find any equivalent fractions to make the denominators the same.)
- Ask selected students to display their responses how to add and subtract fractions with unlike denominators, such as \(\frac{2}{3}+\frac{1}{2}\).
- “Comparado con lo que hicieron en los otros problemas, ¿en qué fue diferente su estrategia para encontrar esta suma?” // “How was your strategy for finding this sum different than the other problems?” (For the first one, I had thirds and thirds so could just add them. For the second one, it was thirds and sixths so I just had to change the thirds to sixths. Here I had to change both the thirds and the half to sixths to get parts of the same size.)
- “¿Por qué tener un denominador común nos ayuda cuando sumamos o restamos fracciones?” // “Why is having a common denominator helpful when adding or subtracting fractions?” (When the parts all have the same size I can just add or subtract the number of parts.)
Lesson Synthesis
Lesson Synthesis
“Hoy comparamos distintas estrategias para sumar y restar fracciones” // “Today we compared different strategies for adding and subtracting fractions.”
Display: \(\frac{2}{5}+\frac{3}{10}\)
“Describan cómo encontrarían el valor de esta suma” // “Describe how you would find the value of this sum.” (I would use tape diagrams for fifths and tenths. I would use a number line using 10 as a common denominator. I would break each fifth into two equal pieces which are tenths and then add \(\frac{4}{10}\) and \(\frac{3}{10}\) to make \(\frac{7}{10}\).)
Consider giving students time to record their answers in a math journal before they share their thinking.
Cool-down: Suma y resta de fracciones (5 minutes)
Cool-Down
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