Lesson 13
Meaning of Exponents
Let’s see how exponents show repeated multiplication.
13.1: Notice and Wonder: Dots and Lines
What do you notice? What do you wonder?
13.2: The Genie’s Offer
You find a brass bottle that looks really old. When you rub some dirt off of the bottle, a genie appears! The genie offers you a reward. You must choose one:
 $50,000; or
 A magical $1 coin. The coin will turn into two coins on the first day. The two coins will turn into four coins on the second day. The four coins will double to 8 coins on the third day. The genie explains the doubling will continue for 28 days.
 The number of coins on the third day will be \(2 \boldcdot 2 \boldcdot 2\). Write an equivalent expression using exponents.
 What do \(2^5\) and \(2^6\) represent in this situation? Evaluate \(2^5\) and \(2^6\) without a calculator.
 How many days would it take for the number of magical coins to exceed $50,000?
 Will the value of the magical coins exceed a million dollars within the 28 days? Explain or show your reasoning.
Explore the applet. (Why do you think it stops?)
A scientist is growing a colony of bacteria in a petri dish. She knows that the bacteria are growing and that the number of bacteria doubles every hour.
When she leaves the lab at 5 p.m., there are 100 bacteria in the dish. When she comes back the next morning at 9 a.m., the dish is completely full of bacteria. At what time was the dish half full?
13.3: Make 81

Here are some expressions. All but one of them equals 16. Find the one that is not equal to 16 and explain how you know.
\(2^3\boldcdot 2\)
\(4^2\)
\(\frac{2^5}{2}\)
\(8^2\)

Write three expressions containing exponents so that each expression equals 81.
Summary
When we write an expression like \(2^n\), we call \(n\) the exponent.
If \(n\) is a positive whole number, it tells how many factors of 2 we should multiply to find the value of the expression. For example, \(2^1=2\), and \(2^5=2 \boldcdot 2 \boldcdot 2 \boldcdot 2 \boldcdot 2\).
There are different ways to say \(2^5\). We can say “two raised to the power of five” or “two to the fifth power” or just “two to the fifth.”
Glossary Entries
 cubed
We use the word cubed to mean “to the third power.” This is because a cube with side length \(s\) has a volume of \(s \boldcdot s \boldcdot s\), or \(s^3\).
 exponent
In expressions like \(5^3\) and \(8^2\), the 3 and the 2 are called exponents. They tell you how many factors to multiply. For example, \(5^3\) = \(5 \boldcdot 5 \boldcdot 5\), and \(8^2 = 8 \boldcdot 8\).
 squared
We use the word squared to mean “to the second power.” This is because a square with side length \(s\) has an area of \(s \boldcdot s\), or \(s^2\).