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A certain substance decomposes according to a continuous exponential decay model. To begin an experiment, the initial amount of the substance is 7600 kg. After 13 hours, 1748 kg of the substance is left.

(a) Let [tex]t[/tex] be the time (in hours) since the beginning of the experiment, and let [tex]y[/tex] be the amount of the substance (in kg) at time [tex]t[/tex]. Write a formula relating [tex]y[/tex] to [tex]t[/tex]. Use exact expressions to fill in the missing parts of the formula. Do not use approximations.

[tex]y = \square e^{(\square) t}[/tex]

(b) How much of the substance is left 23 hours after the beginning of the experiment? Do not round any intermediate computations, and round your answer to the nearest whole number.

[tex]\square[/tex] kg


Sagot :

Let's solve this step by step.

### Part (a): Find the formula relating [tex]\( y \)[/tex] to [tex]\( t \)[/tex]

We are given that the substance decomposes according to a continuous exponential decay model. The general formula for exponential decay is:

[tex]\[ y = y_0 e^{kt} \][/tex]

where:
- [tex]\( y \)[/tex] is the amount of substance left at time [tex]\( t \)[/tex]
- [tex]\( y_0 \)[/tex] is the initial amount of the substance
- [tex]\( k \)[/tex] is the decay constant
- [tex]\( t \)[/tex] is the time in hours

We need to find the value of [tex]\( k \)[/tex].

We are given:
- The initial amount [tex]\( y_0 = 7600 \)[/tex] kg
- After 13 hours ([tex]\( t = 13 \)[/tex]), the remaining amount [tex]\( y = 1748 \)[/tex] kg

Using the formula [tex]\( y = y_0 e^{kt} \)[/tex]:

[tex]\[ 1748 = 7600 e^{13k} \][/tex]

First, we solve for [tex]\( k \)[/tex]:

1. Divide both sides by 7600:

[tex]\[ \frac{1748}{7600} = e^{13k} \][/tex]

2. Take the natural logarithm (ln) of both sides:

[tex]\[ \ln\left(\frac{1748}{7600}\right) = 13k \][/tex]

3. Solve for [tex]\( k \)[/tex]:

[tex]\[ k = \frac{\ln\left(\frac{1748}{7600}\right)}{13} \][/tex]

This results in:

[tex]\[ k = -0.11305199769684167 \][/tex]

Therefore, the exact formula relating [tex]\( y \)[/tex] to [tex]\( t \)[/tex] is:

[tex]\[ y = 7600 e^{-0.11305199769684167 t} \][/tex]

### Part (b): Find the amount of substance left after 23 hours

We need to find the amount of substance left after [tex]\( t = 23 \)[/tex] hours. We use the formula we derived:

[tex]\[ y = 7600 e^{-0.11305199769684167 \times 23} \][/tex]

Let's compute it:

[tex]\[ y = 7600 e^{-0.11305199769684167 \times 23} \][/tex]

Evaluating the exponent:

[tex]\[ -0.11305199769684167 \times 23 = -2.600195947 "*"\][/tex]

So we get:

[tex]\[ y = 7600 e^{-2.600195947} = 564.368597244732 \][/tex]

Rounding to the nearest whole number:

[tex]\[ y \approx 564 \][/tex]

Therefore, the amount of the substance left after 23 hours is:

[tex]\[ 564 \, \text{kg} \][/tex]

In summary:
- The exact formula relating [tex]\( y \)[/tex] to [tex]\( t \)[/tex] is [tex]\( y = 7600 e^{-0.11305199769684167 t} \)[/tex].
- The amount of the substance left after 23 hours is approximately 564 kg.
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