Name______________________________________________________________________________Date_____________Period_________________A#_C-‐13_ Exponential Growth: The Duckweed Task Duckweed (or lemna minor) is a type of plant that multiplies fast. The number doubles every day. A pond that has this plant can get completely covered in a short time. The surface then looks like green grass. We will look at the area (in m2) covered by the plant instead of thinking about numbers of individual plants. Beginning with 1 m2 of plants on one day (starting time t = 0), after 1 day (t = 1) there are 2 m2 of it. The formula that describes this growth process is A(t) = 1 ⋅ 2t 2 1. t is the time measured in days, A is the area covered in m2, and 1 area (in m ) 2 is the initial value of the area (in m ). What does the 2 represent in the formula? 2. The graph at right shows this process for the first four days. To the nearest tenth of a day, at what moment will there be 3 m2 of duckweed in the pond? 6 m2 ? 12 m2 ? 3. Now without the help of the graph: At what moment would you predict there would be 24 m2 of duckweed? 4. The same question for 5 m2, 10 m2 and 20 m2 time (in days) In this activity we will focus on the inverse relationship between time and area. That is, instead of looking at the function A(t) above, we will look at its inverse, t(A). It seems strange, because time is usually the independent variable. But there are applications where an amount of a substance can be measured and that amount indicates how old an object is. For example, that is how the age of objects can be calculated using the carbon dating method. The relationship between time t and covered area A can be described with tables. 5. Complete the empty cells in the following tables using the information from questions 2–4: A (m2) 1 2 4 8 16 32 t (days) 0 1 A (m2) 3 6 12 24 t (days) 1.6 A (m2) 2.5 5 10 20 t (days) 2.3 A (m2) 0.25 0.5 1 t (days) 0 6. What is the pattern you used in the above tables to find values that were not given in the graph?
Below is a more detailed table with values accurate to two decimal places: A 1 2 3 4 5 6 7 8 9 10 11 12 13 t 0 1 1.58 2 2.32 2.58 2.81 3 3.17 3.32 3.46 3.58 3.70 A 14 15 16 17 18 19 20 21 22 23 24 25 26 t 3.81 3.91 4 4.09 4.17 4.25 4.32 4.39 4.46 4.52 4.58 4.64 4.70 7. You can find several patterns in the table above. Some of these were already explored in the tables you completed in question 5, such as: a. If the area is doubled, time increases by 1 day. Find an example of this in the table above and describe it below. b. When A is an integer power of 2 (like 1, 2, 4, 8, 16, ...) you can find exact numbers for t. Find an example of this in the table above and describe it below. 8. Since we are looking at t as a function of A, you can think of t(3) as “the time it takes to grow from 1 m2 (the starting value) to 3 m2 .” Using this notation, you can find other values in the table, like: a. t(3) + t(2) = t(6) (Find these values in the table and verify that this is correct.) b. t(5) + t(5) = t(25) (Find these values in the table and verify that this is correct.) 9. The values in the table are rounded to two decimal places. So, sometimes when you add two values, the outcome may be slightly different from what you may expect. a.) Look for more examples of patterns in the table like those described in #8. b.) Try to explain the patterns that you notice. 10. What will be t ( A) + t (B ) for any areas A and B? 11. We can also find a pattern when we subtract two values: a.) Why should this be true: t (6) − 1 = t (3) ? (Hint for what value of A does t(A)=1?) b.) Find the results for: t (14) − t (7) t (30) − t (5) ; c.)What will be t ( A) − t (B ) for any positive values of A and B?
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