**Video Transcript**

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Transcript
**Exponential Growth and Decay**

The largest longhorn breeder’s competition is coming up and Old MacDonald is tired of coming in second place. His wife, Mrs. MacDonald, suggests that Old MacDonald create a **formula** using **exponential growth** and **decay** to feed his favorite longhorn, BEVO, so that his horns grow faster.

### Exponential Growth

Old MacDonald's wife reminds him that the standard form of a **growth equation** is: **x_t = x_0 * (1 + r) ^{t/n}** and to be very careful with his calculations. 't' is the time in days, so ‘x sub t’ is the final size after ‘t’ days. ‘x naught’ is the original size, (1 + r) is the

**growth factor**, where '1' represents 100%, and ‘n’ is the time it takes for the horns to grow by a full growth factor.

Old MacDonald comes up with a special vitamin mix that promises a horn growth of 25%, which can be written as 0.25, every 10 days. So the formula is: x_t = x_0 * (1 + 0.25)

^{t/10}. The initial length of BEVO’s horns, 'x naught', is 4 feet. In order to know how fast BEVO’s horns will grow, Old MacDonald plugs in the number of days for ‘t’. So, after 10 days, the equation for how long BEVO’s horns will be is 4 * (1.25)(10/10) feet, or simply, 5 feet.

Every day for 50 days, Old MacDonald plans to feed BEVO meals with specially-prepared vitamins mixed in. Let’s take a look at how much BEVO’s horns will grow: After 10 days, BEVO’s horns will be 5 feet long. After 20 days, they will be 6.25 feet long, and after 50 days, BEVO’s horns will be about 12.21 feet long and BEVO will be assured to win the competition!

### Exponential Decay

BUT Old MacDonald makes a SLIGHT **calculation error** and writes 2.5 instead of 0.25. After just 20 days, BEVO’s horns are already at 49 feet! Since Old MacDonald isn’t allowed to enter a longhorn with horns longer than 12.5 feet into the competition, he has to **reverse** the process. But he only has 30 days left in which to do so. Old MacDonald’s wife helps him again and tells him that the **equation for decay** is similar to the equation for growth, BUT instead of adding 1 and 'r' to get the growth factor, 'r' is **subtracted** from 1 to get the **decay factor**.

Old MacDonald wants to have BEVO's horns close to 12.5 feet, so he concocts another vitamin mix that has a **decay factor** of 37%, which means that every 10 days, BEVO's horns will shrink by 37%.
BEVO's horns currently measure 49 feet from tip to tip. Let’s take a look at the length of the horns after 10 days and see if this new pill will fix Old MacDonald’s problem. After 10 days, BEVO’s horns should be 30.87 feet long; after 20 days, about 19.45 feet and 12.25 feet after 30 days.

It’s the day of the longhorn breeder’s competition! Old MacDonald is so excited! WHAT’S THIS!?!? I guess you could say Old MacDonald is “stumped” as to how this happened…

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