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Ch. 8 - Techniques of Integration
Hass - Thomas' Calculus 15th Edition
Hass15th EditionThomas' CalculusISBN: 9780137616077Not the one you use?Change textbook
Chapter 8, Problem 8.5.70

Solve the initial value problems in Exercises 67–70 for x as a function of t.
(t + 1) (dx/dt) = x² + 1 (for t > -1), x(0) = 0

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Rewrite the given differential equation \((t + 1) \frac{dx}{dt} = x^{2} + 1\) to isolate \(\frac{dx}{dt}\): \(\frac{dx}{dt} = \frac{x^{2} + 1}{t + 1}\).
Recognize that this is a separable differential equation. Rearrange terms to separate variables \(x\) and \(t\): \(\frac{dx}{x^{2} + 1} = \frac{dt}{t + 1}\).
Integrate both sides: \(\int \frac{dx}{x^{2} + 1} = \int \frac{dt}{t + 1}\).
Evaluate the integrals: The left integral is \(\arctan(x)\), and the right integral is \(\ln|t + 1| + C\), where \(C\) is the constant of integration. So, \(\arctan(x) = \ln|t + 1| + C\).
Use the initial condition \(x(0) = 0\) to find \(C\): Substitute \(t=0\) and \(x=0\) into the equation to solve for \(C\), then express \(x\) explicitly as a function of \(t\) by taking the tangent of both sides.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Separable Differential Equations

A separable differential equation can be written so that all terms involving one variable are on one side and all terms involving the other variable are on the opposite side. This allows integration of both sides separately to find the solution. Recognizing and rearranging the given equation into separable form is essential for solving it.
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