# Calculus

Poli 761 (a fictitious substance) decays by about 7% every day. How much of a 88 pound sample remains after 9 days?

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asked by Ann
1. I thought it would just be A = 88e^(0.07*9), which equals 165.23 pounds, but apparently that's not correct...

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posted by Ann
2. You can't always just plug in e for the base of the exponent. After 1 day, .93 remains. After t days, .93^t remains. So, we have

88 * .93^9 = 45.80 lbs

Now, if you want to use e as the base, recall that .93 = e^ln(.93) = e^-.0725

So, .93^t = e^(-.0725t)

In your formula, you have the amount growing, since e^.07 is greater than 1.

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posted by Steve
3. ok thanks

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posted by Ann

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