The national debt of a South American country t years from now is predicted to be D(t) = 70 + 18t4/3 billion dollars. Find D'(8). D'(8) = 1 X Interpret your answer. The national debt is increasing Find D'(8). D"(8) = X Interpret your answer. by The rate of growth of the national debt is increasing X billion dollars per year after 8 years. by X billion dollars per year each year after 8 years.

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Author:James Stewart
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Chapter1: Functions And Models
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### National Debt Growth Prediction

The national debt of a South American country \( t \) years from now is predicted to be \( D(t) = 70 + 18t^{4/3} \) billion dollars. We need to determine \( D'(8) \) and interpret its meaning.

#### Step 1: Finding \( D'(8) \)

To find the derivative of the debt function \( D(t) \):
\[ D(t) = 70 + 18t^{4/3} \]

Using the power rule:
\[ D'(t) = \frac{d}{dt} \left(70\right) + \frac{d}{dt} \left(18t^{4/3}\right) \]
Since the derivative of a constant is zero:
\[ D'(t) = 0 + 18 \times \frac{4}{3} t^{\frac{4}{3}-1} \]
\[ D'(t) = 18 \times \frac{4}{3} t^{\frac{1}{3}} \]
\[ D'(t) = 24t^{\frac{1}{3}} \]

Substituting \( t = 8 \) into the derivative:
\[ D'(8) = 24 \times 8^{\frac{1}{3}} \]
Since \( 8^{\frac{1}{3}} = 2 \):
\[ D'(8) = 24 \times 2 = 48 \]

#### Interpretation of \( D'(8) \):

The national debt is **increasing** by **48** billion dollars per year after 8 years.

#### Step 2: Finding \( D''(8) \)

To find the second derivative of the debt function \( D(t) \):
\[ D'(t) = 24t^{\frac{1}{3}} \]

Using the power rule again:
\[ D''(t) = \frac{d}{dt} \left(24t^{\frac{1}{3}}\right) \]
\[ D''(t) = 24 \times \frac{1}{3} t^{\frac{1}{3} - 1} \]
\[ D''(t) = 8t^{-\frac{2}{3}} \]

Substituting \( t = 8 \) into the second derivative:
\[ D''
Transcribed Image Text:### National Debt Growth Prediction The national debt of a South American country \( t \) years from now is predicted to be \( D(t) = 70 + 18t^{4/3} \) billion dollars. We need to determine \( D'(8) \) and interpret its meaning. #### Step 1: Finding \( D'(8) \) To find the derivative of the debt function \( D(t) \): \[ D(t) = 70 + 18t^{4/3} \] Using the power rule: \[ D'(t) = \frac{d}{dt} \left(70\right) + \frac{d}{dt} \left(18t^{4/3}\right) \] Since the derivative of a constant is zero: \[ D'(t) = 0 + 18 \times \frac{4}{3} t^{\frac{4}{3}-1} \] \[ D'(t) = 18 \times \frac{4}{3} t^{\frac{1}{3}} \] \[ D'(t) = 24t^{\frac{1}{3}} \] Substituting \( t = 8 \) into the derivative: \[ D'(8) = 24 \times 8^{\frac{1}{3}} \] Since \( 8^{\frac{1}{3}} = 2 \): \[ D'(8) = 24 \times 2 = 48 \] #### Interpretation of \( D'(8) \): The national debt is **increasing** by **48** billion dollars per year after 8 years. #### Step 2: Finding \( D''(8) \) To find the second derivative of the debt function \( D(t) \): \[ D'(t) = 24t^{\frac{1}{3}} \] Using the power rule again: \[ D''(t) = \frac{d}{dt} \left(24t^{\frac{1}{3}}\right) \] \[ D''(t) = 24 \times \frac{1}{3} t^{\frac{1}{3} - 1} \] \[ D''(t) = 8t^{-\frac{2}{3}} \] Substituting \( t = 8 \) into the second derivative: \[ D''
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