Ordinary differential equations

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SUMMARY

The discussion centers on demonstrating that the function φ(x) defined by the equation (φ(x) - tan(x))/(φ(x) + cot(x)) = e^(∫(tan(x) + cot(x)) dx) is a solution to the differential equation y'(x) = 1 + y(x)^2. The integral of (tan(x) + cot(x)) is solved as -ln|cos(x)| + ln|sin(x)|, leading to the simplification e^(-ln|cos(x)| + ln|sin(x)|) = tan(x). The participant encounters difficulty in isolating φ(x) from the resulting equation and seeks assistance in managing the algebraic manipulation required to solve for φ(x).

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  • Understanding of ordinary differential equations (ODEs)
  • Familiarity with integration techniques, specifically integrating trigonometric functions
  • Knowledge of algebraic manipulation and solving equations
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  • Study the method of solving ordinary differential equations using separation of variables
  • Learn advanced integration techniques for trigonometric functions
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Students studying calculus, particularly those focusing on ordinary differential equations, as well as educators seeking to enhance their teaching methods in solving differential equations and integrating trigonometric functions.

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Homework Statement



Show that φ(x) defined by,

(φ(x) - tan(x))/(φ(x) + cot(x)) = e^(∫(tan(x) + cot(x)) dx

is a solution of the differential equation y'(x) = 1 + y(x)^2

The Attempt at a Solution



Solving the right hand side first,

∫(tan(x) + cot(x) = ∫(tan(x)dx + ∫cot(x)dx = -ln|cos(x)| + ln|sin(x)|

e^(-ln|cos(x)| + ln|sin(x)|) = sin(x)/cos(x) = tan(x)

So,

(φ(x) - tan(x))/(φ(x) + cot(x)) = tan(x)

And here's where I get stuck. I cannot solve for phi. I just end up getting lost in the algebra.
 
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Assuming your work so far is correct, aside from the fact that the absolute values disappeared in the ln terms, and the constant of integration is missing, you have this:

(φ(x) - tan(x))/(φ(x) + cot(x)) = tan(x)
Multiply both sides by (φ(x) + cot(x)):
φ(x) - cot(x) = tan(x) * (φ(x) + cot(x))

Multiply the right side, and then get both terms involving φ(x) on one side and all other terms on the other side. Factor φ(x) from the terms containing it, and divide both sides by the other factor.
 

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