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### Course: 7th grade (Illustrative Mathematics)>Unit 5

Lesson 13: Lesson 13: Expressions with rational numbers

# Expressions with rational numbers

Learn to compare expressions with positive and negative fractions. Created by Sal Khan.

## Want to join the conversation?

• I know Sal is working through solving each answer to see if it equals -2/3, however with the initial question being that, shouldn't we eliminate examples one and three right away as being positive regardless of their values?
• Yes, if this were an exam question, then that would be a very sensible shortcut. Here, I think Sal was more interested in showing examples of working with rational numbers rather than simply getting the answer.
• is a fraction rational or irrational?
• Rational. A rational number is a number that can be represented by the fraction of two integers. So, fractions are naturally rational. Hope this is helpful! :-)
• was that the bite of 87
• yes, ofc it was
• If we have one fraction with a negative numerator plus a fraction with a negative denominator, do we simply pretend that both fractions have negative numerators? For example, if our equation is -5/3 + 2/-3, should our answer be -7/3?
• I personally tend to think about the negative sign being before the fraction, like -(5/3) -(2/3). I think having it in the numerator is also acceptable, but it probably shouldn't be in the denominator. And yes, the answer to the example equation would be -7/3.
• what is a rational number?
(1 vote)
• A rational number is a number that can be written as a fraction, for example, 2/1, or 4/5. It doesn't matter if the fraction can be turned into a whole number or not. In comparison, an irrational number is one that is a recurring decimal with no repetition, eg pie, or 5.624319678.
• at Sal said that -1+ -3= -4, but we learned that a negative + a negative = positive, not negative.
• he said that negative times negative = positive but a negative added to another negative is still a negative
• my class is unstable
• same help
(1 vote)
• Why is the first problem no? is it because its positive?
(1 vote)
• Yes, positive and negative numbers are not the same even though they might be the same "number." Difference between having 7 dollars in your pocket and owing 7 dollars with nothing in your pocket.
• I know Sal is working through solving each answer to see if it equals -2/3, however with the INITIAL and MAIN question being that, shouldn't we eliminate examples 1 and 3 right away as being positive(+) regardless of their values?

If you are ever doing these in a test/exam, I would most CERTAINLY use the most EFFICIENT method to eliminate what we ALREADY know about positives(+) and negatives(-) so that I don't waste TOO much time in calculating those that are obviously WRONG! Since a test/exams are timed.
(1 vote)
• Sal was just taking his time so that he could help people understand how to do this.