standard_description: Solve multi-step real-life and mathematical problems posed with positive and negative rational numbers in any form (whole numbers, fractions, and decimals), using tools strategically. Apply properties of operations to calculate with numbers in any form; convert between forms as appropriate; and assess the reasonableness of answers using mental computation and estimation strategies. For example: If a woman making $25 an hour gets a 10% raise, she will make an additional 1/10 of her salary an hour, or $2.50, for a new salary of $27.50. If you want to place a towel bar 9 3/4 inches long in the center of a door that is 27 1/2 inches wide, you will need to place the bar about 9 inches from each edge; this estimate can be used as a check on the exact computation.
problem_activity: Warm-up
The purpose of this warm-up is for students to reason about expressions with negative exponents on a number line. Students explore a common misunderstanding about negative exponents that is helpful to address before scientific notation is used to describe very small numbers.
For students’ reference, consider displaying a number line from a previous lesson that shows powers of 10 on a number line.
Launch
Give students 2 minutes of quiet work time, followed by a whole-class discussion.
Student Facing
Kiran drew this number line.
[IMG]
Andre said, “That doesn’t look right to me.”
Explain why Kiran is correct or explain how he can fix the number line.
Student Response
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Activity Synthesis
The important idea to highlight during the discussion is that the larger the size (or absolute value) of a negative exponent, the closer the value of the expression is to zero. This is because the negative exponent indicates the number of factors that are
\(\frac{1}{10}\)
. For example,
\(10^{\text-5}\)
represents 5 factors that are
\(\frac{1}{10}\)
and
\(10^{\text-6}\)
represents 6 factors that are
\(\frac{1}{10}\)
, so
\(10^{\text-6}\)
is 10 times smaller than
\(10^{\text-5}\)
.
Ask one or more students to explain whether they think the number line is correct and ask for their reasoning. Record and display their reasoning for all to see, preferably on the number line. If possible, show at least two correct ways the number line can be fixed.


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