Exponential Growth and Decay

Doctors use iodine-131 to treat some forms of cancer. The half-life of iodine-131 is 8 days. A patient receives a treatment of 12 millicuries. A millicurie is a unit of radioactivity. How much iodine-131 remains in the patient 16 days later?

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  1. I know the answer is 3, but I want to use a function, since it is the main focus of the lesson. I having trouble setting up a function.

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  2. if the half life is 8 days, 1/2 is gone every 8 days. so after 8 days it is 1/2, 16 days is 1/4...

    so at 16 days it is 12 x 1/4 = 3 mc

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    posted by Elina
  3. Another way to solve it is:

    amount = 12 (1/2)^(168)
    = 12(1/2)^2 = 3 millicuries

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    posted by Elina
  4. How did you get 168?

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  5. And how did you get 2?

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  6. That was a typo. She meant 16/8
    since that is how many half-lives there were.

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    posted by Steve
  7. Oh, that makes more sense. Thanks, Elina and Steve!

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