Adding and subtracting fractions with negatives | 7th grade | Khan Academy | Summary and Q&A

TL;DR

Learn how to subtract fractions with unlike denominators by rewriting the problem as addition and finding a common denominator.

Key Insights

• ❓ Subtraction of fractions requires rewriting the problem as addition and finding a common denominator.
• ❓ Remember that subtraction does not follow the associative property.
• 🪜 Adding the opposite of a fraction is equivalent to subtracting that fraction.
• ❓ Finding a common denominator involves determining the least common multiple of the denominators.
• 🎭 Carefully perform the operation step by step to obtain the correct result.
• ❓ It is important to understand the concept of opposites when subtracting fractions.
• 🫥 Practicing with number lines can help visualize the subtraction of fractions.

Transcript

• [Instructor] Let's say we wanted to figure out what 3/7 minus negative 7/3 minus 11/3 is. Pause this video and see if you can have a go at it before we do it together. All right? Now let's work on this together. And you might be tempted to deal with the negative 7/3 and the 11/3 first because they already have a common denominator. But you have t... Read More

Questions & Answers

Q: Why do we rewrite the subtraction problem as addition?

By rewriting the problem as addition, we can add the opposite of the second fraction, which makes it easier to find a common denominator and perform the operation.

Q: Can we use the associative property when subtracting fractions?

No, subtraction does not follow the associative property, so the order of operations is crucial. We must be careful not to combine fractions before finding a common denominator.

Q: How do we find a common denominator for the fractions?

To find a common denominator, determine the least common multiple of the denominators. Multiply each fraction by a factor that makes their denominators equal.

Q: Why do we add the opposite of a fraction when subtracting?

When we subtract a fraction, we add the opposite of that fraction to another fraction. This is because subtraction is the same as adding the opposite.

Summary & Key Takeaways

• When subtracting fractions with unlike denominators, rewrite the problem as addition by adding the opposite of the second fraction.

• You cannot use the associative property when subtracting fractions, so be careful with the order of operations.

• Find a common denominator for the fractions and perform the subtraction.