### 4.3 Exponential Decay

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• Math Help

To make a spreadsheet similar to the one in Example 3, use the steps below. [Note that Step 6 uses the half-life formula given in Example 3, .]

1. Enter the titles "Time, t" and "Amount, A" into row 1.
2. Enter 0 into cell A2.
3. Enter the formula =A2 + 3 into cell A3.
4. Select cell A3. From the Edit menu, choose Copy.
5. Select cells A4 through A10. From the Edit menu, choose Paste.
6. Enter the formula =500*0.5^(A2/3) into cell B2
7. Select cell B2. From the Edit menu, choose Copy.
8. Select cells B3 through B10. From the Edit menu, choose Paste.
9. Select cells B2 through B10. From the Format menu, choose Cellsâ€¦.
10. In the Format Cells dialog, select the Number tab.
11. From the Category: list, select Number.
12. For Decimal places:, enter 2. Then click OK.
• Consumer Suggestion

Did you know that every two seconds someone in the U.S. needs blood and more than 38,000 blood donations are needed every day? One donation can save the lives of up to three people.

There are eight different blood types with the most common being O-positive. A supply of each blood type is needed by hospitals. Blood types typically in short supply are O-negative and AB-positive.

• Checkpoint Solution

You can estimate how much longer it will take for there to be less than 1 milligram of the drug in your bloodstream by entering the half-life formula

You can see that the amount of the drug drops to below 1 milligram after about 27 hours. About
2 milligrams were left after 24 hours so this takes about 3 hours longer.

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