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Exploring Exponential Growth North Carolina Council of Teachers of Mathematics 43 rd Annual State Conference Christine Belledin The North Carolina School of Science and Mathematics Durham, NC

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GOALS FOR THE SESSION We will show how to use data about grain production and population growth in Uganda to compare linear and exponential growth. We will show how students can understand the meaning of the constants in an exponential function by relating them back to our context.

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WHERE THIS IDEA COMES FROM… Reverend Thomas Robert Malthus ( ) British cleric and scholar Known for theories about population growth and change.

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MALTHUSIAN THEORY

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FACTS ABOUT HUNGER Total number of children that die each year from hunger: Percent of world population considered to be starving: Number of people who will die from hunger today: Number of people who will die of hunger this year: 1.5 million 33% 20,866 7,615,360

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Grain production for Uganda in 1000’s of tons Year Grains

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BELOW IS GRAPH OF THE DATA We would like to build a linear model for the data set.

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LINEAR FUNCTION Y=61.255x

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USING OUR LINEAR MODEL Interpret the slope and intercept in context. Make predictions about future food production. Later compare growth of food production to population growth.

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POPULATION GROWTH FOR UGANDA To the right is a table of Uganda’s population in millions in the years from 1995 to Year Population

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CREATE A SCATTER PLOT OF THE DATA

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CONSIDER VARIOUS MODELS Linear Quadratic Exponential

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FROM PREVIOUS WORK We know Linear growth is governed by constant differences. Exponential growth is governed by constant ratios. Let’s use this knowledge to find a model for population...

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ANOTHER OPTION: RE-EXPRESSING THE DATA We can re-express the data using inverse functions. If we think the appropriate model is an exponential function, let’s use the logarithm to “straighten” the data. Consider the ordered pairs (time, ln(population)). Look at the graph of this re-expressed data.

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COMPARING GROWTH Can we find ways to compare growth of food production to population?

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EX. 2: FOOD PRODUCTION VS. POPULATION GROWTH 1.The population of a country is initially 2 million people and is increasing at 4% per year. The country's annual food supply is initially adequate for 4 million people and is increasing at a constant rate adequate for an additional 0.5 million people per year. a.Based on these assumptions, in approximately what year will this country rst experience shortages of food? Taken from Illustrative Mathematics

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FOOD SUPPLY VS. POPULATION CONTINUED… b. If the country doubled its initial food supply and maintained a constant rate of increase in the supply adequate for an additional 0.5 million people per year, would shortages still occur? In approximately which year? c. If the country doubled the rate at which its food supply increases, in addition to doubling its initial food supply, would shortages still occur?

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WHY ARE THESE PROBLEMS SO POWERFUL? Students see that mathematics can help us understand important real-life issues Students have the chance to create mathematical models. We can help students make sense of the constants in the models. (Interpret constants in context.) Students build tools to help them distinguish between different types of growth based on mathematical principles.

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CCSS CONTENT STANDARDS HSF-LE.A.1HSF-LE.A.1 Distinguish between situations that can be modeled with linear functions and with exponential functions. HSF-LE.A.1aHSF-LE.A.1a Prove that linear functions grow by equal differences over equal intervals, and that exponential functions grow by equal factors over equal intervals. HSF-LE.A.1bHSF-LE.A.1b Recognize situations in which one quantity changes at a constant rate per unit interval relative to another. HSF-LE.A.1cHSF-LE.A.1c Recognize situations in which a quantity grows or decays by a constant percent rate per unit interval relative to another. HSF-LE.A.2HSF-LE.A.2 Construct linear and exponential functions, including arithmetic and geometric sequences, given a graph, a description of a relationship, or two input-output pairs (include leading these from a table)..

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MORE CCSS CONTENT STANDARDS S.ID.6 Represent data on two quantitative variables on a scatter plot, and describe how the variables are related. b. Informally assess the fit of a function by plotting and analyzing residuals. Represent data on two quantitative variables on a scatterplot, and describe how the variables are related. c. Fit a linear function for a scatter plot that suggests a linear association. S.ID.7 Interpret the slope (rate of change) and the intercept (constant term) of a linear model in the context of the data.

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CCSS MATHEMATICAL PRACTICES 1.Make sense of problems and persevere in solving them 2.Reason abstractly and quantitatively 3.Construct viable arguments and critique the reasoning of others 4.Model with mathematics 5.Use appropriate tools strategically 6.Attend to precision 7.Look for and make use of structure 8.Look for and express regularity in repeated reasoning

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RESOURCES FOR TEACHERS NCSSM Algebra 2 and Advanced Functions websites See Linear Data and Exponential Functions Link to NEW Recursion Materials NCSSM CCSS Webinar: Session 1: Using Recursion to Explore Real-World Problems

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MORE RESOURCES Illustrative Mathematics Tasks that illustrate part F-LE.A.1.a F-LE Equal Differences over Equal Intervals 1 F-LE Equal Differences over Equal Intervals 2 F-LE Equal Factors over Equal Intervals The Essential Exponential by Al Bartlett

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LINKS TO DATA AND INFORMATION Gapminder World Hunger Map Link Link to Data for Uganda My Contact Information: Christine Belledin – NC School of Science and Mathematics For copies of the presentation and other materials, please visit after Monday, November 4. Thank you for attending!

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